I've certainly found that the fat/thin issue isn't as clear cut with rats, as there are so many differences in body, size etc.
For example, my older boy Jake (nakie) has been a constant source of querying statements "he's too thin! why can you see his bones"? Well, it's just him. He is a naturally sticky rat and he eats fine and his body weight, usually, is reasonably constant (although now he's getting on and less active he's starting to pile on the grams o.o)
Others, for example Ivy, have been huge from 8 months onwards. She looks mahoosive (big hips!) but she, again, maintains a constant weight that is within average limits (kinda), she's perfectly active and - so far - has developed no rolls.
I would say the best way to tell is to know the average weights for girls and boys (325-450g (girls) 450-650g (male) according to AFRMA) and if your rats surpass these averages (or you notice worrying signs prior)consider them overall - e.g, is there rolls? do they drag? what do I feed them? etc and on the other side, are they ill? what is their appetite like? is it vet check up time?.
Keeping a monthly guide of your rats weight can tell you if they're increasing/decreasing and if a trend if noticed (aka going up and up and up) once they have reached full maturity, you can tweak their diet/excercise regime accordingly. It'll also be a good indicator of illness