In all reality I think it was pretty easy to tell what was real and what was CG. A good indication, to me actually, was a lot of the camara moves would be almost impossible to do in reality. Thus, CG is probably the way some things were shot. Maybe composited some folks in for good measure.

Now if you look at Attack of the Clones, then that's an entirely different story. ILM blew me away with numerous shots I thought were real. Like the scene with Mace Windu and Yoda, sitting in a room with blinds shadowed over them. All completely CG, except Mace Windu. Truly awesome!