How can i tell if one of my female rats is pregnant?

she looks alot more plump in the belly area and was with the male in a serperate cage for a few days and he apperantly mounted her...my friend wanted snake feeders, but ive been told that cage mates wont breed, so i have seperated her fro the remaning female and male..i know that thier gestation is only 20-23 days so i guess i will have to wait? should i cover the cage so that there is no disruption so she will not be stressed and nervious and eat her young?