In addition to being a driving hazard, underinflated tires wear the sides of the tire and add to your gas bill. Overinflated tires, on the other hand, wear more quickly overall and have an increased potential for a blowout. It's suggested by the DMV to keep tires no more than five pounds per square inch below or above what is recommended for your tires, but how do you know?
You can check your tires in the comfort of your own driveway, but it's a great idea to do the procedure right in front of your local gas station's air pump. And most offer free air with purchase of gas. It's best to check air pressure first thing in the morning while the tires are cool. Warmer air expands, so you won't get a correct reading.