You should check your tire pressure at least once a month. Your tire pressure can affect your gas mileage, tire life, and the safety of you and your passengers. Tires are a very big part of your vehicles safety equipment.
To find your vehicles recommended tire pressure just check the owners manual for your vehicle. For vehicles 2003 and up you should find this information on the placard. The placard is located on the "b pillar" which is the driver side doorjamb. Generally, if that area is too narrow it should be next to the drivers seat on a surface facing inward. You should look for placards in older vehicles in areas like rear passenger doorjamb, fuel filler door, glove box/console door, or the engine compartment. The tire information placard should have the vehicle's weight capacity, the recommended inflation pressures for the tires, and the original equipment tire sizes(this includes the spare tires).
Here are some tips on tire pressure:
We all have been guilty of only paying attention to a low tire, but remember your vehicle has four tires so check all of them.
Check your spare tire every few months or so. For "space saver" spares you can use the tire pressure rating on the sidewall.
If you're going on a trip check your tire pressure with a quality gauge. Your eyes are incapable of measuring a tires pressure.
The most important tip, understand how temperature can affect tire pressure readings. Before your vehicle has been driven, first thing in the morning is when your tires are their coldest. Checking your tire inflation pressure should be before the temperature rises and before any direct sunlight on the tires.
Image credit: ThenAndAgain, garrydolley
Information: tirebuyer, google
I did not know this stuff hum