Chinchillas are relatively easy to sex, a matter of days old or as adults. Some find it harder to tell when they are younger & I will admit it does take practice to realize the difference at young stages of life. My advice is, if you aren't a 'pro' yet, wait a week or two before trying to decide what it is since they can look a lot alike the first week or so.

Its pretty easy once you have some guidence & know what to look for. Here are some helpful pictures.

Males: With males its VERY easy to know what your looking at. You won't see testicals until the male is about 4 to 5 month of age, so don't be looking for them.  The young male kits genitals look a lot like the females, but there is about a half inch to an inch of space between the anus & the penis. Same with the adult you just have the testicals that are also present.

Chinchilla Kit Male 

Chinchilla Adult Male

Females: They are a BIT different than a male. Females have no space between the anus & vagina. Take a look at the pictures for help.

Chinchilla Kit Female

Chinchilla Adult Female