Majority of the time I don't call foot faults unless it's completely obvious. Like if they're taking a giant step into the court. If it's a couple of inches I don't see how you'd be able to tell from the opposite side of the court while still concentrating on the return. There was one time in mixed doubles where the female was serving from the wrong side of the court by a few inches (she'd been on the center line and stepping to the the deuce side before serving it to the deuce court). In that situation I stopped the ball and informed her and just let her play a let since I didn't want to be a jerk about it.