ORDER PARTS - Order a Pressure Gauge to have it on hand to replace the broken Pressure Gauge. If you want to match the pressure gauge to the one above with a dial, see "Pressure Gauge w/ Dial Hayward Oem Back Mount". The dial allows you to set the upper psi limit at which you should backwash your filter.
CHECK READING - Check the pressure reading on the Pressure Gauge. It should read about 10-15 psi. When the pressure moves up to 10 psi or so higher, it is time to backwash your filter.