Before buying my house I had a pool service person check it out, which included pressuring the underground pipes only to check for leaks. He had a guage and ran a good test to verify if there were leaks there. That was several years ago and I can't recall the exact method, ie. did he plug the outlets, etc.

May be worth calling around and see if there is an experienced pro that can run such a test. I'm lucky to have access to a good one, that only does service and doesn't have a storefront so he isn't promoting "solutions" I don't need.