rounded edges, where a crisp, original edge has been overpolished is the #1 clue. non-flat (wavy looking, from overpolishing) surfaces are another.
another main clue is "smeared" looking screw holes, where polishing has removed material from one side or edge of a screwhole, kinda ovalizing it.
flattened pins, that should be proud.
one of the main things i look for is crisp stampings. when a rollmark or stamping is made, it almost always "raises" the metal around the letter, or mark. it takes very little polishing to remove this "edge". lots of folks look for shallow stampings, but you have to be careful. s&w, as well as colt had very variable stamping "depth" across their histories, as rolls and stamps wore.
sometimes you can even just tell, when it doesn't "look" right. best thing is to look at lots of guns, and become familiar with what the originals look like. once you become familiar with stuff you are interested in, you can spot differences quickly.
that is the stuff i usually look for. sometimes it is painfully obvious, and sometimes a really nice job is pretty hard to detect.
good luck!