How do you know if a dog is pregnant?

Our intact female has a little bulge in her stomach... she had her heat in March so we're thinking she's not pregnant, or if she is she has only one pup. The bulge is very, very small, barely noticeable and it is not because she is fat. She was never left alone during her heat but it is conceivable that we some how made an error and an intact male had access to her. How can we tell if she's pregnant without going to the vet?