How can you tell if a female cat is spayed? I read that it's so hard that even some vets can't tell until they start the operation and realize that the cat is spayed.

I was just wondering, because there is this female cat on my street, and I noticed that her "behind" looks much more different then Sara's (Not like I go around looking at cat's butts or anything.. ) So I was just thinking about it and wondering if this cat is actually a female, or maybe a neutered male???