Yup, before you really learn what your looking for, the best way to learn is to hold 2 rat butts side by side in the palm of your hand, lift their tails and judge the distance between the anus and genitalia.

Once you figure you have a male (longest distance from anus to genitalia), it's easy to just compare side by side until you can tell at just a glance.

My males are usually very prominent by 3 weeks of age.