Presta valves, also called French valves, are found on high-performance bicycles, as opposed to the Schrader (automotive style) valves found on touring bikes. Usually one checks the tire pressure through a Presta valve by just attaching a pump with a pressure gauge. However, you may have a flat tire on the open road and need to know that you filled your spare tire with enough air to avoid a pinch flat and having to hitchhike home. Fortunately, digital pressure gauges are now available for Presta valves for only $10. They are small, portable and fit inside a jersey pocket.
Remove the valve cap. The valve cap is there not to keep air from coming out but instead to keep the point of the valve from piercing the spare when rolled up.
Unscrew the built-in valve cap, a "captive" nut that cannot be removed.
Tap the end of the valve to release a little air. This ensures the valve is not stuck.
Push the chuck of the pump nozzle or the pressure gauge down on the valve far enough to cover part of the straight section of the outer valve. Make sure you do this vertically so that you don't bend the valve core. Read the pressure directly off the gauge, if you are not using a pump gauge. If you are using a pump, proceed with the next steps.
Flip the pump's lever to clamp down on the tire valve. Whether you flip the lever up or down depends on the make of your pump.
Pump up the tire a few strokes to increase the pressure inside the pump to the pressure inside the tire. Once the gauge needle stabilizes and you sense pumping resistance has peaked, the gauge is accurately indicating the tire pressure.
Flip the pump's lever back to the original position and pull the chuck off gently in a direction parallel to the valve so that you don't trigger air release from the tire. You'll hear some air escape, but this is from the pump, not the tire.