The M2A2ODS and M3A2ODS and M2A3 and M3A3 are the current Bradley variants. M2 (and "Ausf.'s") are infantry fighting vehicles (carry more GIBS, or guys in back sleeping), M3 (and "Ausf.'s") are cavalry fighting vehicles and carry fewer men and more TOW missiles. There are some structural differences in the rear ramp, top cargo hatch and back deck. Seating layout is also quite different, M3A2's(and later variants) don't have any firing ports on the rear ramp and crew access door.
Visually, an M2 and an M2A1 look similar. Same with an M3 and M3A1. The main difference between the M2/M2A1 and M3/M3A1 is the side firing ports, M3-series doesn't have any. Once you go to the A2 variant and beyond, they look very different from the earlier versions and even the most casual observer could tell the difference if they were next to one another.
As far as an M2A2 and M3A2, they are very similar in appearance and unless you looked at the back side or the top of the hull rear, you wouldn't know the difference. Of course the bumper numbers would be a sure give away.