How do i check the tyre pressure of my car?

How bloke y times during the year are you supposed to check the tyre pressure of a car? is there a recommended amount of air that should be in the tyres? are there any devices that can help?
Answer
I would recommend once a week. buy a tire pressure gauge from the store. those gauges that you see at the refueling stations on those units next to the gas station are usually dropped so man y times i would not trust the accuracy. check the owners man ual for the recommended tire pressure. the reading on the sidewall is the max pressure and i would not recommend that. some man ufactures put a placard on the drivers door jamb or int he glove box.