Technically they don't get pregnant, they're egg layers and not easy to breed in captivity anyway. If you have one and it's looking chubby, it's more likely to have a gut load of internal parasites or have over-eaten.
If you look right above the butt. If that little area is black and the stomach is bloated. She will have the eggs in there 1st then if fertilized then she will be pregnant. Then I can't remember how...