As far as I know if it's a female you can't tell by looking if it is spayed or not. I think I heard that vets usually look for a scar on the belly to see if a female cat has been spayed or not. And on a cat that isn't bald and sitting on your lap I imagine it would be hard to find that scar. I think if the "behind" looks different it is possible that it's a neutered male.