How do I know if my female rabbit has really been spayed?

Okay I recently bought a female rabbit from a lady that could no longer care for her due to health reasons. Now she had told me that she got her from the Humane Society, which then I would ume my rabbit (Nalla is her name) would be spayed, right? But she humps all her stuffed animals, one bear about her size especially, and she is our only rabbit. And also I was petting her, she loves to have her back scratched she lick my free hand the whole time I scratch her back, cute huh! Anyway that's not my point, got a bit off track, anyway I was petting her and was patting her very fluffy little tale and all of a sudden she lifts her rear end as she would to a male rabbit I would ume. So would she still do this even though she has been spayed? And she is about 3 years old and I again would ume she had been spayed quite some time ago because there aren't any visible surgery scars or any sign that is has been done recently. So what makes her so..... "horny" I guess would be the word to describe her actions! Oh and we have a neutered male cat ( fixed many, many years ago) and a spayed female dog. Now Nalla just loves our dog to death and always wants to cuddle and snuggle up with her even though Sammy (our dog) isn't a small dog she is a mutt and about the size of a Lab, but Nalla just always wants to cuddle with her and that's all but the cat, now there's another story! Nalla, when ever she is near the cat, the first thing she does is mount the poor cat and hump his brains out! Which of course since rabbit bite when they hump, the cat flees for his male cat pride! So why love the dog and hump the cat? Oh Nalla is a Holland Lop and the cat it just a bit bigger than she. So all these lifting (presenting?), stuffed animal and cat dog loving questions! Can some one please help me out here?? I'd greatly appreciate it!! Thank You - Kris