Look for a rear derailler hanger, always a good sign, components tell a story, cotterless cranks probably good (especially the star-shaped stronglight variety), cottered steel cranks probably not good, alloy rims probably good, steel rims (especially Rigida "Chromalux") probably not great. Most of these french bikes had Normandy high flange hubs, so they do not tell you much. Leather saddle made by Ideale (or even Norex, made by Ideale), probably good, cheap plastic seat, not good. You see how it goes. Most important to the overall case with be frame tubing, which may or may not be obvious (sometimes decals disappear over time!). Generally a "named" tubing (ie Reynolds, Columbus, Vitus, Durifort, etc) is very good, even if it is just the 3 main tubes (trois (3) tubes renforce, in french), no-name (usually a number) not so good.
Just have to view and evaluate. Lots of cheap Gitanes around, and a few good ones. Good luck!