Dealers hole card
When you are sitting at 1st base and if the casino is using one of those checking devices with the red / green lights to check for blackjack, if the dealer is jamming his cards into the reader when he has a ten up and check for an ace under, he can bend the tens. From 1st base you will start to see the ten value cards (when they are the dealers down card) take on an upward curl on the corner. Seeing the slight upward curl on the dealers down card comes in handy when he has an ace on top and asks for insurance, etc.
You wouldn't see it at every table but when you do, take advantage of it.