I think you would look more at the behaviour of the rabbit. It can be hard to look at external signs since you don't really know what to look for and if you have nothing to compare it with it would be really hard to know. Also since most of the stuff would be inside, you can't really see if a little bit might have been left once it is all closed up and healed.
Some behaviours to look for would be more normal hormonal behaviours. Excessive humping, especially with a rabbit who lives alone (pairs can hump, but it is more for dominance than a mating thing). Males tend to spray if not neutered, so spraying can be a sign as well. Courting behaviour with people such as running circles around your feet, trying to hump you and a buzzing sound when they do it. Some rabbits are more moody at times as well. If you did notice him to be really hormonal before getting him neutered and he hasn't changed much and it has been a few months, then that could be a big sign. Of course if he ends up fathering babies after about 6 weeks, then he would not have been neutered right, this is not a sure way as they could have removed the sperm producing parts but still left some of the hormone bits.
If you do suspect something, talk with your vet or even get another vet from the one who neutered him to take a look.
