The degus are a little more "tricky" than gerbils for gender identification. Indeed, the sexual dimorphism (The set of more or less marked morphological differences between male and female individuals of the same species. Wikipedia) of degus is almost zero. The male's testicles are inside the body and the only way to differentiate a male from a female is by the distance between the anus and the urethra, as well as the size of the latter.

The advantage is that the difference is apparent from birth!

We can see here a female on the left and a male on the right