Rat or mouse? I can always tell with mice, I lift them up by their tail and press my finger gently on their spine, and if theres a crease normaly those have proved to be pregnant.
- Matt
Come here little guy. You're awfully cute and fluffy but unfortunately for you, you're made of meat
I usually just wait until the female starts to plump up. Although I will admit, I was surprised by the last 2 litters I had. I kinda forgot I left the male in with the girls.