There are three versions of the call:
1) The server’s toe (or more) touches the baseline before he strikes the serve,
2) The server’s front or back foot cross the imaginary line BEHIND the service line created by the edge of the “T”
3) And an even rarer one… the server stands beyond the singles sideline (in singles) or doubles sideline (in doubles).
Roving USTA referees will usually warn senior tournament players that either they are “watching for foot faults” (which usually means they see it happening; so watch out) or they will warn the server to “watch his feet” before making the call on him.
In club play, it is the call of the receiver to make against the server. And in singles, that is almost impossible to do, if you are trying to return serve.
In one crazy interpretation, a New Hampshire friend was playing in a USTA league championship and was called for a foot fault BEFORE HE EVER SERVED. According to the referee, he was in “his serving motion” and his foot was touching the line; so he called it even before the serve was made.
In my opinion, that call is premature. He hadn’t served yet; therefore he cannot foot fault.
On the call in general, if I were a real tennis purest, I would say that a foot “slightly touching the line” is the same as a ball “being just out.” But, it really doesn’t bother me if my opponent’s toe is constantly on the line when he serves.
What DOES bother me is the net rusher who gets an unfair advantage by getting a running start toward the net, with half his foot over the line on every service motion.
One frustrated friend of mine once took extreme action when he couldn’t get someone to stop foot faulting. When it was his serve, he stood five feet INSIDE the service line and served an ace. The opponent of course challenged him; but my friend said, “it is just a matter of degree.”