I think one thing that might help is to place some kind of marker on face down enchantments after an incantation or enchantment is played on the target and another when an attack is made. This way, you will be reminded to flip them face up if they are triggered and later, you will have evidence that the chance for trigger has passed if they attempt to reveal it later.

Of course, this is not fool proof as someone wishing to cheat could tell the judge that whatever they are claiming is false. This is something that is unavoidable. If someone wants to lie, there is nothing anyone can do about it.