First - check with the previous owner if they had an entire male (or a male kid - they are fertile pretty much from 3 months old) - if the answer is yes - I would presume the female is in kid
If no luck with previous owner, then normally I would think a scan at the vets, but not possible at the moment really. So just wait and see... feed normally (not too much though), make sure they have got exercise and read up on kidding/lambing.
What type/breed are they?