Also, don't forget to look for the recomended tire pressure on the side of the tire. One more thing, if you use one of those air pumps at a garage where it costs you twenty five or fifty cents, they almost always measure in metric but they do have a dual scale on the end of the pump.
You can also buy a pressure gauge at Wal Mart. Another way to check is just to examine your tires by sight and feel only (by kicking them). This method works fairly well, although it's not as accurate as using the gauge.
The best way is to own tire pressure gauge - it can help you know if you do not have enough air or if you put too much into your tires. I believe you can buy them at any automotive store such as autozone.