Provide proof of previous trades or prepare to take on a load of questions.
A good way to provide proofs of signals would be for him to post the signal given on that day for the paid group hashed with SHA-256 or some other hash, and then releasing the exact text once the signal isn't good anymore. For example, if the signal was to short DOGE, he could get the message
Short DOGE until price is X, stoploss at X, e.t.c.
post the hash, and then give the unencrypted message after. If he doesn't post the message/deletes his post, it's fair to assume that it was a bad signal.