fulton has a good point and I agree with him. Just press he previous owner (without being a jerk) and try to find out what the story is. Also I'd say if you are looking at a car say with 100k miles on it and an engine bay to match that age, and a TMIC or something that looks like it has 20k miles on it, boom, it was returned to stock. Might not be that easy but look for little things like that. Another example could be upgraded oil lines but everything else is stock... that could mean they returned everything else to stock and left the lines in because they didn't want to deal with it.
Again, easier to say than actually find but that is what I would look for.