The test follows the question.
Eight methods, each with a useful role and practical limits. The investigation may combine several.
Thermal imaging
Read surface temperature patterns in context.
Acoustic listening
Listen for useful evidence along a suspect supply.
Tracer gas detection
Use an appropriate tracing method when the system permits it.
Pressure testing
Ask a focused question of the system or a section.
Moisture mapping
Compare the spread of moisture across accessible surfaces.
Leak noise correlation
Compare signals from suitable points on a supply.
Borescope inspection
Look into a suitable void through limited access.
Dye testing
Follow a suspected water path under controlled conditions.