Leak detection

[drinkwater]

I have a 5 year old undergound pool and I have a leak.
It only leaks when the pump is running and I suspect a pressure side issue in the return lines.
I have been googling about pressure testing the return lines, basically they talk about isolating each individual return line.
But I am missing something here because how can I test each idividual return when there is only one line from the filter which brings water back to the pool through the 3 returns ?
I assume that one line coming from the filter will feed the 3 returns so how I can I test each individual return ?

[Teapot]

This is one for a hydrophone.

[Denniswiseman]

You could put a valve on each line