Use approx a foot length of hosepipe with a tap connector at one end and a trimmed car tyre valve at the other. Cut the valve until it can be forced into the hose by approx 1". Use a Jubilee clip or wrap and twist wire around the hose to secure the valve. Attach to the tap, turn on and use a car tyre pressure gauge to read the approximate pressure (1 bar = 14.5 psi).
Alternatively, to calculate the water head, you should measure the vertical distance from the water surface in the header tank to the tap (1 bar = 33.47 ft of water).