How can i tell if a female cat is prego?

Someone dropped of 2 cats male and female at my house. No vet records ages no info what so ever. The female cats stomach is bulgy and round. I had 2 female cats that were pregnant 1 was my cat that I had since I found her by a street drain the other a stray. This cat is long haired and mine were short haired. How can I tell if this cat is pregnant. I'm keeping the babies if she is pregnant til they are weaned and get a vet check. But how do I tell if she is prego