I haven't done it yet, but that's only because I haven't encountered anyone who was that bad. If I ever do, I will warn him per USTA rules, and if he continues to do it, I'll start calling it on him.
To elaborate slightly, if there is an official available and you believe your opponent is foot-faulting, go get the official.
If no official is available, Darrin is right. You are supposed to warn your opponent that you believe he/she is footfaulting and give them a general idea of what they are doing (politely, something like "It looks to me like your foot is inside the line when you toss" or "I think you're taking a step" etc.)
If the person continues to footfault, you are allowed to call it.
you may also want to doublecheck with the league director (or tournament referee or an official). In lower level leagues and tournaments, they'll sometimes go easy on footfaults just so players can play the game. In some adult beginner and young juniors events, you could call footfaults on almost every serve, but what would be the point?
Doesn't sound like this applies in your case, but I thought I'd give the long answer in case somebody else checks this thread (and this is all per USTA--rules may vary slightly in other parts of the world).