IME The surest way to find out if your bike has french-threaded cranks is to try some 9/16-20 pedals.
This is what Sheldon had to say:
Older French bicycles used a different, smaller diameter where the pedal screws into the crank, 14 mm x 1.25 mm.
The vast majority of French bicycles imported into the U.S. used standard 9/16" x 20 tpi crank threading, but if you find an older bicycle that was bought in France, you may run into this problem. The French-thread cranks can be tapped out to 9/16". This is much easier (and more worth the trouble) with aluminum cranks. Start the taps from the inside of the cranks,so any possible thread damage occurs where it doesn't matter. French pedals will often be marked "G" (Gauche=left) and "D" (Droite=right).
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