I used to really want a borescope. The problem back then was that they were very pricey (and I had less money, more mouths to feed, etc). I recently discovered though that the prices have come way down, so I bought one (on principal) off Amazon for about $40.

Here is some video from sending the scope down the seat tube of my bike, to the area above the bottom bracket. I had just gotten the frame back from being repaired, so being able to inspect the Di2 wiring was both useful, and fun!