Install a pressure plug on the intake valves, skimmers and one on the main drain by disconnecting the covers and screwing on the plug.
Disconnect the unions at the filter and screw the pressure tester to the union. Disconnect one of the plugs and allow the pipes to fill with water to get a pressure reading.
Watch the pressure gauge. When the pressure reaches 15 to 20 PSI turn the valve on the pressure tester to the left to turn it off. Continue to watch the gauge. It should hold pressure. Check the lines for leaks if there is a drop in pressure.
Test each PVC line individually at the pump to find where the leak is so the pipe can be repaired.