Q: dealer's viewpiece

what exactly does a dealer see thru the peephole?   I have read
elsewhere that "the dealer knows exactly what the player does -
weather or not he has a natural."

Wouldn't the dealer also know when he has a 20?  It seems he must.   I
hope so cuz I think I found a dealer who 'tells' . . . when he checked
for BJ I could tell be his mannerism that what he saw was not good for
the players, and sure enough - 20.