When you take your car to the tire shop these days, they check your alignment with laser beams and other sci-fi-worthy instruments. This is great for absolute precision, but if you're trying to determine if you've got an alignment problem in the first place, there are a couple ways to check for yourself -- right in your own driveway.
Check for camber, the tilt of the tire toward or away from the frame of the car, by standing in front of the parked car (or behind it, if you're checking the rear tires). If the tires tip in -- that is, the tops are closer together than the bottoms -- the tread will wear away on the inside of the tire first. If they tip out -- bottoms closer than the tops -- there will be more wear on the outside.
It's also pretty easy to check the toe, or stance of the tires, in the driveway. Think of the tires on one axle, either front or rear, as feet. The fronts of the tires in either case are where the toes would be. Toe in means the fronts of the tires are closer together, like standing pigeon-toed, and the tread will wear from the outside. Toe out means the rear of the tires are closer together, like first position in ballet, and the tires will wear from the inside. The wear from toe-out tires is sometimes called "feathering."
If you notice your tires are slightly off angle or if you see the tell-tale tread wear, it's time to get your tires aligned to increase their usefulness. While you're at it, you might want to get those tires rotated, too, which we'll discuss next.