Critiquing his performance is a major no-no, particularly when you compare it to the performance of a previous partner. Certainly you want to speak up if what he's doing isn't OK with you or is painful, but minor concerns like his breath, manner of kissing or other trivial issue shouldn't be brought up, particularly if it's a casual hookup and things aren't likely to lead to a relationship. Finally, one of the worst things to say during a hookup is anything that makes you look insecure. If you think you're unsatisfying for your partner, then your actions can show it. Self-confidence is sexy and when you question his motives - saying things like, "You're probably thinking of someone else," then it's a major turnoff. The same goes for constantly seeking reassurance. It's one thing to ask him his preferences and how he likes certain things you're doing; it's entirely another to ask, "Am I doing this right?" Read his body language if you're not sure. If he seems to be enjoying what you're doing, then you're doing it just fine.