Sexing chinchillas is very easy, even at birth - though occasionally mistakes can be made. As can be seen from the above examples (left), the females urethral cone and anus are very close together, this gap is generally at most only
1-2mm.  This is generally referred to as there being no gap between them. 

The gap between the males penis and its anus is generally
5-15mm.

In kits this difference is still noticeable, though sometimes it may be necessary to move the males penis to notice the gap between the two due to the way it may sit in very small kits.  With kits it is worth checking at least weekly up to the age of 6-8 weeks to be sure of having it correct - most often males can be mislabelled as females until they have grown more and the gap is more obvious