I hate making this call (though I will when I am absolutely certain) even when I do see it because I know I don't see it every time. I agree that there are some who seem to be dying to call it at least once a game just to show everyone that they are watching closely.I might struggle with that and I hope that others give me better advice if I am wrong. The challenge is that you're watching the ball until the point that it hits the bat. Then you look down and see the foot on the ground. I don't think I would know if the foot had been there for a full second or a 0.1 second so I don't know that I would know the foot's exact location at the time of impact.
I do know that some umpires seem to want to call it at least once per game to prove they are doing their job or something (just like I know of one umpire that will call at least one illegal pitch to prove the point but that also might just be a cynical observation).