we could do that with an EEG scan, and then proceed and give a prescription that will lead to a happy life, some people already do that, albeit they do it not from a scientific perspective.
Ok so we have 2 scenarios:
1st one:
We have the uncertainty of not knowing what true happiness is, and thus whatever we may get close to, may as well fill and take the place of true happiness, in this scenario you could go to the Fisherman (taking him as an sample of someone that claims to be happy and that we both know), and ask him if he "is truly happy with his life", of course as someone that claims to be happy he will say yes, when asked why, he will describe how his current life is, however we don't have any other tools other than to believe in his word.
The problem with this scenario is exactly what you describe, people wont know what they want to do in order to be happy unless they experience it, the problem with this is that people are not all the same, so you can't prescribe a model of happiness to them, people have to find happiness by themselves, and is all up to chance and luck whether they will find happiness or not.
Meaning a person is not assured that they will find a purpose that will make them happy, even if it exist, which means there's the possibility for a person whose purpose that may make them happy doesn't exist, in this sense and before you make a objection, i am talking about wholesome happiness, AKA true happiness, real happiness, etc.
An example of this: a person whose purpose is space exploration (in this thought experiment, we know what the purpose of the person is, but the person itself only has an idea of this), but it lives in a world that has a geocentric view of space, even when Copernicus already demonstrated the opposite, that person could write a fictitious book about men going to the moon, but it would end there, his dreams can't become true, (this has nothing to do with Jules Verne, is just an example of someone whose purposes doesn't exist, it can be more extreme than that, after all our reality has limitations), going back to this person, he could lead another kind of life, maybe that of a fisherman that has a small family and at night plays domino with his friends, drink beer and sings songs, was his life happy?
1) we can't tell because only the person itself knows if he is or not truly happy.
2) Now lets assume that we know, we know that out of all the things the person could do, the one that gives it the greatest amount of happiness is space exploration, meaning that being an astronaut makes this person more happy and more fulfilled, than having a family in a fisherman village. However this doesn't implies that being a fisherman doesn't makes him happy, it just doesn't gives him as much happiness as being an astronaut, the question then comes: is the life of a fisherman making that person happy or is he just content (as in it experiences serenity, but it is not joyous, let alone euphoric http://en.wikipedia.org/wiki/File:Plutchik-wheel.svg while we are on it an euphoric(can be replaced with ecstasy but more intense) life is impossible because we are not in a perpetual state of happiness, similarly when i say "an euphoric life" i mean to use Nozick's model of measurement of happiness, which consist on the summatory of all the happy and sad moments in a person's life all the way to his dead, but more on this on the second scenario) with his life? the answer is content, because we already know what makes this person truly happy, having a family is just a compromise that tries to replace what it can't obtain, luckily for its kids his dad wont try to force them to somehow change the world so he can live his dreams through his kids, which is something that is morbidly common nowadays, thinks like child beauty pageants should be illegal everywhere.
The point thus under the 1st scenario is that only a person can know if they are truly happy for themselves, and if there was a way to peek inside them and see if they are really happy, and then know what will make them really happy, we would find that the person wont have a certain surefire way to achieve true happiness in life, for such a thing is not guaranteed nor granted in life.
Doomroar
Oh sure there must be a person for most people out of the 7 billion humans on the planet, probably... i don think that it all comes down to luck/chances, because being objective while in love seems contradictory.
I see the difference of views, i was approaching this using Maslow's hierarchy of needs http://upload.wikimedia.org/wikipedia/commons/6/60/Maslow%27s_Hierarchy_of_Needs.svg (that list has been revisited plenty of times and changes for example i would remove sex from the base and leave it on the 2nd or 3rd floor, since you can't really die from abstinence thus it is not really a basic need (even if it is physiological), similarly depending on the environment of the person social needs go above or below personal needs, etc).
(i recommend you to read all of this first, links included, and then comment, what comes next is 1 long answer that tries to cover all corners, and the sentences are there just to make it easier to read, but just imagine that this was a normal Nietzlawe text)
Anyway back to this, what most people want is probably simple in essence, find love, have a family spend their time with them, see their kids grow, then they die, most people don't really care about other things outside that spectrum, art and literature is whatever may be trending, their spirituality is whatever they were raised with and they don't question it, and their intellectual facts are whatever was told to them before they became adults, even if the information was wrong.
But that is only one kind of common person, the other one wants an opulent life filled with excess and debauchery, deep down they probably want the exact same thing as the first kind of people, however they long for a taste of the things they never had access to, would becoming rich make them happy? if they know how to manage their money, yes, after all money is just a tool to get things, however chances are that they don't know about that, and are instead worshiping money.
Which means that the average person already knows what they want, and what makes them happy, they already have an idea of what happiness is, just like in the story of the fisherman, which i already said is flawed, for many things, one of them being that they portray a model of life as a general ideal that applies to everybody (meaning they live under the second scenario in which we can know and measure happiness, similarly and thus, happiness doesn't has subjective value, this will make sense further down this comment.)
"How do you know what you want until you know about it/until you find it?"
"Do you think you know what you really want at this point in life?"
Those are basic questions of skepticism from the kind of "how does someone gets to know anything." and more importantly, how do they "really" get to know anything.
http://en.wikipedia.org/wiki/Philosophical_skepticism
The answer to both those questions is no, there's no way to "know" something, there's only a justified belief: http://en.wikipedia.org/wiki/Belief#Justified_true_belief
http://en.wikipedia.org/wiki/Theory_of_justification
http://www.niu.edu/~gpynn/Goldman_WIJB.pdf
And even then we wont be able to know if our belief is not a product of human error, which is, in part, why we have falsifiability http://en.wikipedia.org/wiki/Falsifiability
Ok going back to the 2 questions, the first question says "until you find it" meaning that a person can knew what they want through experimentation (the answer to the second one was no, there is no way to "really" know something), but there is a problem with this, that "want" is tied to a longing for happiness, and happiness is subjective, right? (lets assume for now that it is) which means that only the person itself and no one else, can know if they are happy http://en.wikipedia.org/wiki/Solipsism and this provides the following problem(s), how do we know if we are "truly" happy? this leads to at least 2 things, the first the nature of "legitimate things" that can only be experienced internally, such a thing proves an impossibility of knowledge, because we wont have a way to know what is true and what not, in essence whatever experience that gives us a certain amount of happiness can appear itself as "true" happiness; the second is under the assumption that we can know when others are happy http://en.wikipedia.org/wiki/Philosophy_of_happiness (read the part of Positive psychology, i don't have much characters left, interesting because is a really Humeian (as in David Hume) way to prescribe a happy life, mainly focused on denial/distraction, and a "virtue"/value of gratitude, of course it is highly simplified and doesn't shows all the other things that Hume points on his Treatise), and actually measure happiness http://en.wikipedia.org/wiki/Anarchy,_State,_and_Utopia (in this case the gross total happiness experienced in a life), under this scenario (which is the second scenario that i mentioned) a person wont have to search for a purpose that would make it happy, because we can know when someone is happy, how happy they are, and what will make them happy, (continues...)
Cyberdevil
A lot of people to filter through indeed! :) And a lot of text to filter though below, I'm on it...
You certainly do your research! Me I'm more of a: 'create my opinions based on the circumstances of my life' kind of person (meaning I usually have no data to back up any of what I say), so more of the 'authorative testamony' and 'logical deducation' than empiricism... but it's always interesting reading up on this stuff, specially that first chart, simple; conclusive; didn't take more than a minute or two. :) I hope you're not expecting me to read through ALL those other texts (even though you say all), gotta be at least 100k characters in there. O_o
Mmm, makes sense! Similar to the 'thinking an orange is a favorite fruit if you've only tasted lemons', you would indeed not know what true happiness is until you experience it, and without experiencing it you would not know you want it. So what makes us want more? What level of happiness does it take for us to be content with life... or are those entirely different things (as you mention later)? On that family aspect, I've never felt like my purpose was a family, and I feel that'd be a rather strange notion in the modern world - people thrive on visions rather than relations - though if that's for better or worse is something for another topic...
That part about how 50% of our happiness is genetic doesn't seem to hopeful! But the mind is powerful indeed, should be possible to surpass those puny 40% if you train your brain enough! The power of the mind knows no limits!!