The potential in some are obvious as early as 6 weeks old ... the older they get ... the better you know one way or the other.
If you look at it from above, it should appear similar to a brick not a triangle shape ... the shoulders and hips should be about the same width.
The fur should be thick and stand up with no swirl patters. You should be able to blow into the fur (one quick short puff) and it should close back up into place almost immediately. When you blow into the fur, you should not be able to see the actual skin. The guard hairs should stand up straight and not lay to one side.
A chinchilla reaches it's first prime (in reference to it's fur being at it's best) starting at 6 months and reaching it's peak at 10 months, then repeats this cycle every 8 months.
Proper grooming can help a chin with slight fur problems, but improper grooming can hurt the appearance of a perfect chin.
So grooming counts, too.
I believe Jamie did an article on that in the Chinchilla Club Magazine that is on line and can also be purchased in paper back. It can tell you much better than I can.
Jo Ann
PS Chinchillas having a color that should have a white belly, the belly should be pure white. White fur should have no yellowing to it. There should not be a red cast to any of the fur, regardless of the color. If it is a wrap (one having the same color all over) the coloring should remain the same shade /blend all over (such an an ebony, a tan or a violet wrap).