Behind the plate, this is an easy call. I have called this many times during my 25+ years of umpiring. While standing at shortstop, this is a harder call. If I suspect the pitcher is illegal pitching due to the ball and arm release relationship, I will move towards second for a few pitches to get a better look. I only do this if I am with a newer umpire. The call must be MADE. If the illegal pitch is not called, the newer umpires will say, "If I don't make the illegal pitch call, then why should I?"