An air system doesn't have leaking hydraulic fluid to help find the leak, assuming it is an external leak and not a leak in a valve body or double acting piston seal
A loss in pressure over time aids in finding pneumatic leaks. The system is pressurised and the drop in pressure is measured. There is always an allowance and the same with hydraulic systems. Leakage can be ignored if the drip from the source takes a measured time period to develop, otherwise, the component needs changing or the 'o' ring, which in most hydraulic or otherwise leaks it normally is. All this stuff is stipulated in the appropriate maintenance manual. This stuff is engineering 101, the very basics, but such things are usually checked during line maintenance operations.