Aside from taking her to the vet it is hard to tell until she is near ready to give birth. My female gave birth April 19th and we didn't know for sure that she was. We suspected it because she looked bigger and because she was constantly laying on her side and sleeping a lot more often even during the hours she normally would be full of energy. Just keep a close eye on her if she is you will see a change in her that let's you know. If you do suspect she is try not to handle her because it could hurt the kits. Good luck!
