Ummm how do I put this delicately LOL. Aric if they are breeding size rats there is no way you can't "tell the difference". Male rats are rather ummmm "blessed" in that department LOL.

Getting a female already pregnant from the pet store may not be the bargain you believe. They do sound like a decent pet shop but just be aware that sometimes when females breed in a busy pet store environment like that where they are with a lot of other rats, they are stressed and don't have the best of litters. Make sure this female is by herself so she can de-stress and not compete for anything. Lots of peace and quiet, tons of water and food and bedding to make her nest with. Don't worry but do be aware she may not deliver as successfully as a female that is bred in a more controlled manner.

If she's like most rats she will get quite large. You may see her start to sprawl over on her side or even do the "dead rat" thing. Females very full with a large litter will sleep full on their backs and it's sort of startling...they appear dead! LOL They are fine...just close to delivery. Usually about 24 hours before they drop their litters they will build huge nests though I have some females here that don't build a nest until after they have delivered all their pinks.

Have fun! Rat breeding is a lot of fun, a bit of work but very rewarding when you can see the top quality food you've raised for your snakes.