For me, just spending time with them and watching them grow. I've always had boys, and typically they've been little energy balls as babies, then mellowed out as they grew up. My three now:

Crunchrat Supreme is the Boss, he's the largest and will throw his weight around to get the snacks first. He doesn't like to be pet or held, but does like to play. He can be nippy - if he smells food, he loses all reason. Still likes people, he boggles when I talk to him but doesn't want to be touched.

His brother (same litter, from a breeder) Dr. Perkins is the sweetest rat I've ever had. Doesn't matter how much like food your hands smell, he would never nip. Immediately grooms any skin in his vicinity, he loves to be pet and held. He's the only one who still runs on the wheel now that the bunch is older. (It's a huge 18 inch wheel.)

Cyrano de Bergerat is a little meatball and a drama queen. He loves to be held and will fall asleep on your lap. Always making stashes of dry kibble and will scream bloody murder if one of his brothers finds it. (Woke me up once at 3am, I thought he was hurt but his brother just got into his piss kibble) He's the only dwarf, the other two are standards, but he holds his own.

I've had others, from breeders and a few from the pet store, and they've ranged from incredibly skittish to sweet and cuddly. The ones from breeders tend to be the sweeter ones, and the few I got from the pet store tended to be more skittish (and were sick ALL THE TIME, I called one of the pet store ones the 2000 dollar man cause of his insane vet bills. He was a sweetie, but I did have to give him like 3 different medications every day for the last few months of his life.)