The one from the rear sprocket side will have a shiny band where the rubber seal has been rubbing on it. The other side shouldn't have.
If you still need confirmation of collar lengths, I can have a rummage in junk boxes over the weekend, should still have the originals somewhere as I never throw anything away. My current ones were machined from a lump of 316 stainless.
Also if I remember right one, of the rear bushes has a center groove, probably as a marker so the factory knew which one was which.
Regards
Dave