[waltjp]

Jason, like it or not, if the opposing coach points out that a player is not wearing the required equipment and he participated in the previous play I don't see how you can't enforce the penalty. The coach was probably told by one of his own players that the opposing player was lacking the piece of equipment. The coach has now informed you that he suspects that the equipment is missing and your inspection confirms it to be true.

Are you going to tell the coach that he's right but you're going to ignore the rule and not penalize?

It's not an enviable position to be in but the opposing coach was asked before the game if all of his players were properly equipped. Presumably, he said 'yes'.