The best way to tell is by belly checks. All snows generally have some type of defined belly check.
Snow belly:
You can see the faint clear to white checkers.
Avalanche have no belly checks. The belly is a uniform white.
Avalanche belly:
Plain belly with zero color variance.
The thing that will throw it is motley.... a snow motley will have the same stark white belly. When motley seems to be present, I gauge it by the amount of yellow. Generally an avalanche will have little to no yellow, where a snow might have a tinge or a lot.
Avalanche face:
No yellow to be seen.
It all comes down to knowing and trusting the breeder.