I think passion is more important than prowess. I look for women who can move, can dance. But I know that's not a good indicator, I have had a magnificent lover who was not a dancer at all, she blew my mind, and my theory.
I think a good indicator of sexual prowess is confidence. There is a certain kind of confidence that comes from skill, and it's undeniable and unmistakable. At least, once you have experienced it.
Not only the ability to please your lover, but the ability to make it enjoyable for your lover to please YOU.
Let's say a gal is sucking on my dick, I will let her know just how good she's doing: "That's good baby, that's real good...you look so hot with my cock in your mouth"
Positive feedback is important, let that lover know just how much you are enjoying their touch.
And as Laurel says, confidence. Without it you're not gonna have much sex at all.
I am following the same suit here...confidence. If you are comfortable within your self, I think it really shows how you react to others...if you're not happy inside it shows.