Leak detection can only be done on sealed, pressurized pipes, not open-ended pipes. Pro’s will pressurize pipes with suspected leaks using CO2 gas and use sensitive listening equipment to track down hissing noises caused by the escape of the CO2. For really small leaks they can use helium gas and helium leak detector (that’s how they found a really small leak in my water mainline caused by a very poor plumbing job). For open ended pipes all you can really do is shut down all the pumps and wait for the water level to stop dropping. If you know the leak is in a return line you can try to use a borescope camera to look for a leak but that’s not going to help with small leaks, only gross fractures or breaks in a line.
Leak detection requires the use of lots of different techniques to find a leak. It’s not something you can just pickup a tool for and do on your own. People that do it well, have lots of experience and know what to look for. It’s worth paying for the service if there is a knowledgeable professional in your area.