I was hoping someone more knowledgeable and with direct experience could help you, but I see this has not occurred in all these many days! Would it perhaps be possible, e.g. with existing valves, bypass piping or during a shut-down period etc. to bulkhead off and hydrostatically test the pipelines involved (e.g. at a high enough pressure to maybe at least give some sort of feel for the pipeline and future needs?) If the line does not blow up but leaks, perhaps leaks could be located as in new construction (though I understand leaks are generally much harder to find with non-metallic)? Of course, there would probably need to be some sort of repair or contingency plans in case line did blow up in high-pressure testing! There are also now specialty testing firms that advertise ability to find even quite small leakage (that might not be easily found from the surface particularly with pipes that don’t carry sound well) by various means including passing devices through the length of the pipelines, but I suspect this may not be cheap.
Finally, as various soil conditions and/or contamination can degrade both types of pipes you talk about e.g. from the outside, perhaps you could consider some physical potholing or other daylighting to examine/peck on? outside wall conditions in areas? I would however caution that these kinds of pipes can be hard to locate, and perhaps particularly even dangerous/vulnerable to excavate/work around for this and other reasons, and of course all utility and OSHA rules etc. should be complied with in this regard.
