You'll need a piece of chalk, a tire gauge, a pen and a note pad. Make a chalk mark across the tread as pictured, on one front and one rear tire, and then drive a quarter mile or so in a straight line. Stop and study the chalk marks and note the pressure readings on the gauge.
This tire shows over-inflation, having relatively intact marks at the shoulders while the center of the line is more worn. Ideally, the chalk would have faded evenly across the tread surface. Let out some air and try again.