All right, first you find a nice, quiet room and you put on some tranquil music. You have them sit down, relax, take a few deep breaths, then you slam the core book on the table, direct a lamp light in their face and ... wait, no, maybe not. Perhaps you should just sit them down and ask them a few simple questions. You could email these to them but you won't be able to question their responses and they're likely to answer them pretty off the cuff (if the answer them at all).
Oh yeah, and you should give them pre-warning and let the players opt out of it if they want. Some people just want to come over and play and are perfectly happy with your game, or happy enough to complain during the game, and they don't want to be psychoanalysed as part of their gameplay experience. This is particularly so if they don't trust you much because you've steamrolled over their hopes and desires beforehand. Let's face it, we've all done that in our haste to craft the perfect story, haven't we?
So, you sit them down and you ask a few questions. Keep them open-ended. Write down the responses. Don't counter their arguments. In fact, your only responses should be more non-judgmental questions to clarify responses or gain more information. Don't argue with them.
Some good questions are:
"What have STs done before in a game that really annoyed you?"
"What have been your best moments in a RP game?"
"What made you want to quit the game beforehand?"
"What anecdote did a friend tell you that made you really wish you were in their game?"
"What's your favorite videogame genre? Why?"
"What attracted you to WoD / D&D / Pathfinder? In other words, why this game? Why not another one?"
"What are some of your favorite moments in my game?"
"What are some of the most teeth grinding moments in my game?"
You notice how I don't ask what the best and worst parts of my game are? That's because wording it that way will either net you some hurtful criticism ("I hate your stupid NPCs. They're way too arrogant") or will cause them to clam up. By talking in terms of moments, you can both kid yourselves that you're only talking about single instances, and they're more likely to be honest about it.
I would suggest giving no more than 10 major questions UNLESS your game really needs more or you have a player who will quite happily be quizzed for hours. Generally, the only time when you need the equivalent of a Gaming Personality Inventory is when you're running a game that taps into their deepest psychological fears - and even the average horror game should avoid that as most players don't want to delve into their traumas around their peers.