Years ago on this site I learned about "The Chalk Test"
I just got done searching for it and came up empty. Basically it went something like this.Take a piece of chalk and make a stripe across the width of your tire tread.Drive the car in a straight line for (100 feet, yards I really don't remember the exact distance),then check your stripe. If the chalk wore off evenly, you have correct tire pressure. If the chalk wore off in the center only, then your tire is over inflated. If the chalk wore off on the outside edges, then your tire is under inflated. To make it easier to find the stripe I made a mark on the sidewall with the chalk.