Somebody asked the same ...

Somebody asked the same question last week. It's mainly governed by how readily you can access the pipe - through a valve, fitting or whatever. Sahara is well proven and involves floating a microphone on a tether down the line. It will pick up the noise of the leak as it passes. Other options include Smartball which is launched into the pipe and then caught / captured further down the line in a catch pit you may need to install. If the pipe is very strategic and has plenty of capacity you could slipline it - put a new pipe inside the old. If you need more capacity you could pipe burst. International Society for Trenchless Technology and its web-site has lots of good detail on how the various systems work.