Imagine that the car ahead of you can't seem to stay in the right lane. It weaves over the center line, nearly drives off of the shoulder, and keeps drastically changing speeds. When you get to a stoplight, the driver stops well before it, then does not start driving right away when it turns green.
It doesn't take you long to realize that the driver is intoxicated. You know how common drunk drivers are, and you know you've spotted one. Now what should you do?
The most important thing is to realize just how dangerous drunk drivers are and to stay as far away as you can. Don't try to get the driver to pull over. Don't even attempt to pass, even though it sounds logical to get around that type of danger. Just stay behind and keep a good buffer zone between your vehicles.
If you can do so without getting too close, figure out the make and model of the car. Then try to read the license plate number and commit it to memory. If you have a passenger in the car with you, have them take photographs or videos of the other vehicle. Then have them call the police. If you're alone, safely pull over to the side of the road and call the authorities yourself. Tell them where the car is, what direction it's going and what identifying information you collected.
These steps help if you see a drunk driver before an accident, but there is a good chance they'll cause a wreck before you realize they're drunk. If you get hurt, make sure you know how to seek compensation.