Intuition and observation. There's no significant change in behavior once the balls are no longer jauntily gracing his back end (unless, of course, we're talking about a stud cat).But.................
can you know for sure?
I can tell you for certain that I did humiliate one of my DSHs years ago. He didn't bat an eye when he was neutered. But, I used to have him shaved in a lion cut for the summer and not only was he humiliated, he was angry -- for about 4 days. For the first 4 days, he'd stare at me with a very mean squint and hit me every time he walked passed me. He'd mope around with his head hung low and he needed a lot of consolation from my other cat. Then around day 4, he realized he felt a lot better in the heat without the burden of all that fur and I was forgiven. If you find a cat that act's like that after being neutered, I'd say he's a candidate for neuticles.