He can tell, because he has a secret bot detection bot. That bot can detect 100% correctly with no false-positives, as long as he never reveals how it operates. The bot is named ZeroK.

He put it on the blockchain to prove he knows the secret. Can’t be wrong, it’s immutable.

He doesn't need to, he only needs to be confident that someone with more knowledge (or confidence) in the actual status of the commenter won't make a counter-bet