You can try to find a spay scar, but it can be difficult. I can't easily find them on my dogs and with the I don't have to look past a bunch of fur. You would probably have to shave her belly area to check, and even then you might not see it.
It would be nice if there was some sort of hormone test that could be done, either blood or urine. Even if there was, it would probably be expensive. If a person can pee on a stick and find out of they are pregnant or not, why can't a rabbit pee on a stick to see if she is spayed or not.