Attach your pump to the water source using the hose.
Make sure the pump's nozzle is facing as vertical as is possible. You will be measuring the height that it achieves, and this is less accurate a measure of "pump head," the measurement you will be converting from, if the expulsion from the pump is not vertical (and thus the distance it achieves is altered by the difference in gravitational effects).
Open the valve of the pump, and let it run for a few moments, to achieve its standard height.
Measure the distance from the pump nozzle to the height achieved by the expulsion. This number is the "pump head."
Divide the "pump head" in feet by .43 to determine the pump's pressure in psi, that is, pounds per square inch.