Because Italian shows different reflexes of Vulgar Latin phonemes then French does. So if a word is present in French with a phonetic outcome different from mos

Because Italian shows different reflexes of Vulgar Latin phonemes then French does. So if a word is present in French with a phonetic outcome different from most words of Latin origin in French, then you can tell it is a later borrowing. Also sometimes you can trace the slow spread of an Italian word first to the medieval Occitan dialects spoken in southern France, and then only later into French spoken further north.