When a dealer uses a peeking mirror to check the hole care when an Ace or Ten is up, does the dealer know the actual value of the card that is being checked? That is, can a player utilize dealer tells with tables using this peeking device?
The dealer does not know the actual value of the card. Cards are marked on the corners from the factory with a symbol. Only the ten value cards and the aces are marked. When the dealer places the card in the reader then the reader will Indicate If 10 or ace. The dealer cannot be read by tells (anymore). You can purchase playing cards from most Casino gift shops to see examples.
They put the cards in horizontal or vertical depending on if it is an A or Ten. So he only sees the mark if it is blackjack. It is a simple yet effective system. They have been using it for a long, long time.
If that is the case, then Dealer "tells" are possible. However, the new system was brought into existence to overcome this problem. I've never seen a Casino that still uses such outmoded equipment but good luck if you have.