Comment by tenebrisalietum

For example, The iNES format, for NES emulators, has a 16 byte header, not part of the original ROM, that identifies the mapper, screen mirroing if not controlled by the mapper, and whether or not battery backed RAM is included.

NES mapper numbers were standardized among emulator authors. Look for "NES mapper list". Some very early games didn't use a mapper (that's what mapper 0 is).