From what I remember, they blasted holes through mountains and ran the pipe through. Kinda difficult to inspect, I would think. However, there are certainly methods, such as “how much oil did we pump in, and how much came out the other side?”. But given the vast distances and volumes, leaks could be difficult to detect. A decent size leak doesn’t mean there is a small hole, it likely means a large section has corroded.

I’m not defending them, just recounting what I saw.

To answer your question, if certain pieces of my test equipment fail, I’m dead in the water until they’re fixed.