Castrated dogs are still quite capable of mating with a bitch but of course no puppies will result as long as the neutering wasn't recent. As long as your dog has no testicles, I think you can be fairly sure he has been neutered - you don't tend to see obvious scar tissue with a straightforward castration as only a small snip is needed to remove the necessary items
I suppose there is a chance he might have two undescended testicles but if so, he would be infertile anyway - it's more likely he's been neutered as you've been told.
Jane