When the rat shows no fear, let's you pet and scratch it, and doesn't immediately try to run away once you have it out. Especially once it stops pooping on you. Then it's your pet

One of my brown agouti's is like this and he has been a pet since day one. He is a breeder but I have never had plans to feed him off. Even though I have pre killed some old breeders for my retics, I couldn't feed this boy off. He's too cool and totally trusts me.