Guest
We use PEX extensively in in-slab floor heating applications and find testing can sometimes be quite a challenge to the uninitiated. In these situations we may have up to 4,000 feet to test at a time. Hydrostatic testing is usually the quickest and most reliable because leaks will usually be visible, but more importantly pressure drops will show up on the guage quicker.
If freezing is a concern (around here it usually is) then testing with air or other inert gas is the next best option. However this can be dangerous and misleading if proper precautions are not taken. Dangerous simply because compressed air equals more volume (compressed liquid such as water little or no additional volume beyond the capacity of the system being tested) and teh extra volume will literally explode from a pipe rupture or bad connection. Misleading because if you test to say 30 psi when the temperature is 75F and check your test when the temperature has fallen you will show a loss in pressure whether there is a leak or not. If the pressure is zero then start looking for the leak. 
