Printed on the switch is the pressure it closes/opens. You need to install a tee fitting in the line to the switch and connect a manometer or magnahelic to the tee. Turn off the fuel to the unit. Then turn on the unit and read the pressure on the gauge. If it makes the printed units the switch should change state when read with an ohm meter.
If not, the switch is bad. Remember, there is a positive and negative side to all pressure switches.