No heat when you turn it on in the car.

1. Swerving
2. Not maintaining speed
3. Are your lights on?
4. The smell
5. Your speech

Poor performance on takeoff ... rotten gas mileage ... rough idling ... hard starting.

They only way to know is to ask her. This sounds like a useless answer, but it's the truth. There aren't any telltale signs.

Drunk driving is very dangerous and there are some signs that make it pretty obvious that someone is drinking. If you see them driving slow, and swerving, chances are they are also drinking.