As far as I know you can't test for megacolon. If you want to breed her just make sure she breeds to a self (all one color) dark rat (not any white on it). That would lessen your chances for megacolon I would assume...not being a rat genetics expert. I've bred my husky female and I've bred blazed females with no incidents of megacolon in my colony but had I seen even one indication of it I'd have just retired those breeders from the colony. If you are concerned about using her you could just keep her as a pet basically, replace her with another female and she can just keep female feeders company or your other breeder females when they are between litters (that's what I do with my older retired female breeders).

You might want to be very careful about introducing a female in with two mature males even though they are bonded (likely brothers by their looks). I recently had a male rat turn on another when I stupidly and temporarily let both adult males in the very large breeder tank with some females (I was reorganizing the colony and just popped one in for a few minutes with the other and the females). In a flash one male nearly killed the other and these were males that knew each other and normally are fine together. The males may fight each other, or may bite the female arguing over who gets to breed her. Just a thought and something to watch out for with two males in with some females.