I'm no expert, but sometimes, unless there is a visible scar, I think it is hard to tell whether a female has been spayed or not. Hopefully, someone will correct me if I am wrong.

I know that my friends adopted a female puppy from the pound about 6 years ago. At one point, she escaped their fence and was picked up by animal control. They tried to spay her again (new incision) only to realize that she had already been spayed. Luckily, my friends found her up for adoption in the shelter and went through the whole adoption process all over again!

Logan