From the horses mouth when asked how it would be enforced:
"On a complaint basis. A woman would report a man using the facility and a summons could be issued if warranted. Max $ 50 fine."
ANd given the deranged nonce's reference to "anatomically correct" in his legislation ... well ... it seems like a minefield which would only only be legally (as Obama's pbuh executive orders would stop the courts using birth certificates to determine these matters) be resolved by inspecting netherparts.
But it is not before entering the restroom, which is what made the whole thing seem totally nutter. It would be part of a subsequent investigation potentially. That is done from time to time for legitimate reasons. But again, the proposed law is ludicrous.