I had a similar dilemma when asked to check if a stray had been neutered. I looked and he had 2 bumps. Not done? Me supposedly the expert in this situation - expert by association I hasten to add! Well he wasn't behaving like a whole tom so after much consultations decided he had been done.
But I needed to find out what was actually left after a castration. So at the next welfare meeting I went to I raised the question. And yes, very often after a successful neutering two little bumps can be left. Before op 2 round balls after op 2 little bumps. And no, I wasn't embarressed as I think a lot of others were wondering as well!
A quick squeeze is the easiest way to tell, an entire cat will have 2 peas in there and a neuter may have the look of being full but theres nothing to pinch
Not foul proof as they could always be retained lol
^george^ had always kept the look of being well endowed until he got a good grooming and it was all old hair
he lost his street cred in a matter of minutes
MM, if the lump was to the side of his bum then its not whats left post neuter, these would be under it