When my DD was just starting to develop as a pitcher I would take the pocket radar to her lessons. Whatever her top speed was when she was with her PC I would use as the bench mark when we pitched together without her coach. If she was off by 2-3MPH then I knew her mechanics were off.
Last fall she had a back injury that she tried to conceal from us as we were just ready to get started on our fall season. Pocket Radar didn't lie, she was off 3-4MPH on her FB. It's an outstanding tool.