The mirror trick works okay for line of sight stuff, but once you get into bundles or behind cannon plugs, it just turns into a guessing game. I got one after a friend swore by it for checking socket contacts on a gear box, and now I grab it way earlier instead of pulling panels blind. Pair it with a bright headlamp and a steady hand, and you can usually spot the crushed pin or corrosion without having to break the bundle open. It also helps to take a quick photo with your phone pressed to the eyepiece, that way you can zoom in and compare both ends if you're working a intermittent.

Oh wow, that's wild! I actually read a study once about how dust on panels can mess with yield way more than people think, especially in dry areas like Texas. It said something like a 5-7% loss is normal, but if you're near farmland or construction, it jumps up. Kinda makes me wonder if that guy just had a lucky guess or if he's done this before, but either way, it's funny how strangers can spot stuff you miss.