My younger kid is always asking "How did I do?" and I am convinced that he already knows the correct answer. With that said, I don't want to blow smoke and, when appropriate, I point out mistakes and he then explains the why and the where. Some times I do get to simply say "Great!", but I feel as though I have to be honest when he asks. Main problem is probably that he asks when he has done great or not so great - if only he would sulk in silence I would be off the hook.
On a related note, he loves to get better so I find myself making comments about minor improvements. Things like fielding and perhaps getting into a little better position to throw - really little things that could add up eventually. I think he likes these comments, but I sometimes fear that I come across as a little picky. He seems a little more resilient than my older kid (who would sulk quietly) so I guess he doesn't see me as some jerk who only points out the negatives.