A common way to test refrigeration tubing is to pressurize with a gas and watch the bleed-down rate. This can be done with air, a few fittings and a bicycle pump. Pressurize the tubing to 10-15 psig and look for pressure loss over a day. Do not pump to high pressures because of the possible sudden release of pressurized air.
Pex is often tested with water and a gauge to watch the bleed-down rate. Water can be run to high pressures without the worry of explosion, but safety equipment including safety glasses should be worn becaue of the possibility of a pinhole stream hitting vital parts. Do not allow high city water pressure to contunue to be connected during the test as water damage can uccur.