Yeah, it can get down to the gritty, but here is the two-step idiot's guide to thrifting bicycles:
1. If any components (that's brakes, derailleurs, pedals, crankset, hub, seat post) are made by Campagnolo, buy it.
2. If the frame is made of Reynolds 531 or Columbus tubing (there will be a decal somewhere on the frame, usually, but not always, on the seat tube--the one the seat post inserts into), buy it.