If you know the history of the maker you can tell some re-scales.

For example if you take the relatively common Wester Brothers No 34 "De-Fi" razor. The factory scales were celluloid with faux woodgrain, so if you see a De-Fi without a woodgrain set of scales you know it has been re-scaled at some time in its life.

Some rescales that I have found have definite amateur hour look about them, so they're easy to spot too. As others have pointed out a mismatch between the pivot and wedge pins is another tell.

Where it is hard to spot a rescale is when the scales are of the right era and the repinning has been done with some skill.