How do you know if there is too much, or not enough?
Look on the sidewall of the tire. Pressure information is listed in raised letters near the rim. Remove the valve cap from the stem. Make sure the gauge is reset. Press the end with the little gasket which fits the stem. You sill see the plunger rise or the gauge needle move and then come to a stop. Lift off the gauge and read the indicator. Add or remove air as necessary. Reset the gauge and do it again. If both reading are the same, it is likely you held the gauge tight with no leaks. If the reading are different, do it again, as you might not have made a tight seal. You should check and adjust pressure once a month. Do it in the morning before driving the car. Driving flexes the sidewalls which works to heat the tire and raise the pressure because of the heating. If you release pressure because you checked pressure when the tires were warmer than usual, you will have underinflated tires most of the time which is where the 3-4% lost gas mileage Obama talks about comes from.