Pressure testing compressed air pipework is best done after installation and before putting your system into operation.
ALWAYS pressure test the airlines for in wall or underground burial installations before closing up the walls or trench.
You will have to cap each open end of tubing by assembling the wall outlets and letting them hang outside the wall, or use end caps to close off the system.
Plumb in a shut off valve and pressure gauge after the air compressor (this could be a source of leaks).
Bring the system up to pressure, this can be 100-120 psi, close the valve and watch for a few hours. You will notice that the pressure gauge may lose about 5-10 psi and then stay constant. This is due to the compressed air inside the lines cooling down, and stabilizing, which is normal. The air should stay constant overnight with little loss.
If you notice a substantial loss in pressure, check for leaks with soapy water. The tubing joints are typically sound, as they have an o-ring seal, start by checking any of the pipe threaded connections, quick couplers etc.
Remember to assemble all NPT threads with two wraps of teflon tape and then PIPE SEALANT which prevents thread galling.