I would think the easiest method, albeit not terribly objective, would simply be to get someone who is fairly good at manipulation and play out scenarios with them. I've done this a few times as the manipulator, and it's sort of scary how easily I can manipulate people in specific games, even when they know the rules and have witnessed some of my techniques.

If you do try it, I'll comment that time and social pressure help me a lot in making people more pliable, too. I do these as a group exercise, so there's a lot of peer pressure both to perform well, and not to use exactly the sort of "cheats" you should be using to resist manipulation. It's also helped that I've always known the group and thus known how to tweak myself to hit specific weaknesses.

If you find something more useful than this, I'd love to hear it. I've merely learned I'm fairly good at manipulating - I have no clue how good I am at resisting :)