Clark, Holden, Guzan, should be the only ones he's played against in MLS, possibly Edu and Altidore.
Honestly, though, I was just being a bit facetious, as I really don't think Beckham is going to be anywhere near as useful in that regard as the media seems to think. The "big" USA players, like Dempsey, Donovan, Howard, everybody already knows VERY well. Most of the other guys have enough caps to be easily scoutable, and those that don't are likely to be little more than fringe players, and are mostly one-dimensional. It doesn't take any special David Beckham analysis to understand that Robbie Findley is fast but inconsistent in front of goal, or that Edson Buddle uses his size and strength to get into dangerous positions, or that Ricardo Clark is tenacious but has kinda poor technique and tends to tackle too aggressively (as does much of our side).
It's probably just Gomez that Capello will want to know more about, mostly because the rest of us do, too!