casual rounds, even leagues, probably aren't worth calling it for reasons of penalty. In a tournament, especially if it's a matter of close score, you gotta call it.
the first call is a warning, second and beyond is a stroke.
it can be called by an official unsupported, or by a group member with a second. In both cases, must be within 3 seconds of the throw.
Especially with newer players, I'll point it out, saying specifically that I'm not going to stroke 'em for it (or most other reasons...) but they need to be aware of it.
I had a kid, proudly said he's been playing 6 months, tell me the foot fault is not a stroke ever. He also told me he could take a mulligan on an OB since it's legal. He's never read a rulebook. He ran to a buddy of his (this was a St Louis league) who said "yeah, you can take a mulligan". I held out my rule book and asked him to find the word "mulligan". He just cussed me out. Loser. This kid was starting every tee shot from 15-20' behind the tee PAD edge, not the line, and running almost full speed. Throws going every which way, worse than mine even. Foot faults almost every tee shot. But he's been playing 6 moths and knows what he's doing.