Leak detection in a greywater system is likely to be an exercise in detective work using your senses. Because these systems reuse water you cannot use your water meter to detect a leak as described for other fixtures.
Any leak in a greywater system that occurs above ground in plain site should be easy to detect. You will see dripping water and you may smell organic matter. It may be difficult to verify a small slow below-ground leak in a graywater system. If the ground near your system feels squishy and/or there is a distinctive organic odor you likely have a leak. Time to start digging.
Potential areas for leakage include: plumbing connections including connections to any pumps in the system.