In my experience, I have seen more covers with stamp replaced with wrong stamps than I have overpayments.
Easy to tell if you advise the other markings, or manuscript sender directives, on the cover. If French mail, it should have a French entry datestamp. If it has a boxed Aachen Franco mark, it is Prussian Closed Mail, or Bremen / Hamburg the a backstamp of one of those two offices.
Also, the New York exchange office marking would have different numerals that make a determination easy. That is the only way to verify correct rate or correct franking. In general,overpayments require careful examination.
Typical markings associated with the various services may be seen on the exhibit frame shown on my website here:https://www.rfrajola.com/AGX/agx4.htm