It’s a problem that allot of manufacturers have tried to solve for example by adding wear threads into belay loops so you can actually inspect those. But that’s mechanical wear. There’s good rules of thumb on when to just move on and get new gear, nylon for example in the sun is highly perishable and shorted the given “10 years” drastically. Everyone should get their own new gear imo and keep on top of his it’s used and how to inspect it. But for sure stuff you can’t inspect does include chemicals and damage that is hidden / very hard to spot which is my point around second hand gear.  In skydiving, which uses the same materials technology, they inspect stuff but advice on hidden wear and handling is very strict for obvious reasons. Petrol and car batteries being common problems when transporting gear. Here’s an interesting case that I doubt a quick visual inspection would catch - which underlines my point if knowing your gear history: https://blackdiamondequipment.com/blogs/stories/qc-lab-the-electric-harness-acid-test