You can flip your rabbit (making sure that it is well supported when you do this otherwise it will kick and try to flip back over), palpate the abdomen (don't press too hard!) and it feels like she has marbles-like bumps in her abdomen. If so, then she is pregnant. However, if you are not comfortable doing this or want more assurance, I would go to the vet. If you do find out she is pregnant, get her a nesting box.