The 100% proof thing is important. Several years ago, at my previous gym, there was an area rival that "knew" I had an overage kid on my team. They called competition companies we had on our schedule to inform them we were cheating with an overage athlete, posted it on message boards, and basically made our lives a living hell. Want to know what proof they had....the kid in question looked too old to be on the team. She was (and still is) very tall for her age, had just gotten her braces off and is a gorgeous kid. So because they hated us, hated losing to us and had nothing better to do than worry about us, I spent the season getting questioned by untold amounts of people.
Basically, my point is if you don't have absolute proof, shut it. If you have absolute proof, say it.