The following information is quoted directly from the United States EPA.

How do the leak detection methods for pressurized piping work?
Automatic line leak detectors (LLDs)
Flow restrictors and flow shutoffs can monitor the pressure within the line in a variety of ways: whether the pressure decreases over time; how long it takes for a line to reach operating pressure; and combinations of increases and decreases in pressure.
If a suspected leak is detected, a flow restrictor keeps the product flow through the line well below the usual flow rate. If a suspected leak is detected, a flow shutoff completely cuts off product flow in the line or shuts down the pump.
A continuous alarm system constantly monitors line conditions and immediately triggers an audible or visual alarm if a leak is suspected. Automated internal, vapor, or interstitial line monitoring systems can also be set up to operate continuously and sound an alarm, flash a signal on the console, or even ring a telephone in a manager’s office when a leak is suspected.
Both automatic flow restrictors and shutoffs are permanently installed directly into the pipe or the pump housing.
Vapor and interstitial monitoring systems can be combined with automatic shutoff systems so that whenever the monitor detects a suspected release the piping system is shut down. This would qualify as a continuous alarm system. Such a setup would meet the monthly monitoring requirement as well as the LLD requirement.
Line tightness testing

Tracer methods do not measure pressure or flow rates of the product. Instead they use a tracer chemical to determine if there is a hole in the line. With tracer methods, all of the factors below may not apply.
The line is taken out of service and pressurized, usually above the normal operating pressure. A drop in pressure over time, usually an hour or more, suggests a possible leak.
Suction lines are not pressurized very much during a tightness test (about 7 to 15 pounds per square inch).
Most line tightness tests are performed by a testing company. You just observe the test.
Some tank tightness test methods can be performed to include a tightness test of the connected piping.
For most line tightness tests, no permanent equipment is installed.
In the event of trapped vapor pockets, it may not be possible to conduct a valid line tightness test. There is no way to tell definitely before the test begins if this will be a problem, but long complicated piping runs with many risers and dead ends are more likely to have vapor pockets.
Some permanently installed electronic systems (such as some Automatic Tank Gauging Systems) can meet the requirements of a line tightness test.
Secondary containment with interstitial monitoring
A barrier is placed between the piping and the environment. Double-walled piping or a leakproof liner in the piping trench can be used.
A monitor is placed between the piping and the barrier to sense a leak if it occurs. Monitors range from a simple stick that can be put in a sump to see if a liquid is present, to continuous automated systems, such as those that monitor for the presence of liquid product or vapors.
Proper installation of secondary containment is the most important and the most difficult aspect of this leak detection method. Trained and experienced installers are necessary.
See the information on secondary containment for additional information. Secondary containment for piping is similar to that for tanks.
Vapor or groundwater monitoring
Vapor monitoring detects product that leaks into the soil and evaporates.
Groundwater monitoring checks for leaked product floating on the groundwater near the piping.
A site assessment must be used to determine monitoring well placement and spacing.
UST systems using vapor or groundwater monitoring for the tanks are well suited to use the same monitoring method for the piping.
See the information on vapor monitoring and groundwater monitoring for more information. Use of these methods with piping is similar to that for tanks.