No first hand experience, but https://www.ciescorp.net/ is supposed to have a very good generic product
Why not just use a fuel totaliser, JPI etc, it’s what is done in the commercial world. Sure it doesn’t account for leaks, but PFI covers that….Mostly :-)
I have no idea at all, but have never heard of anyone doing it. For sure it would cost thousands, if it were legally possible. There might be a generic American product.
MedEwok, regarding boroscopes for aircraft owners the PUF’s test winner in August edition is Mike Bush’ Vividia VA-400 which sells for about $ 199,— …
No medical quality required, except if you want to determine whether the fuel in your tank is Avgas or Jet A 
The later TBs have highly accurate capacitive fuel gauges, which is an adequate solution although a “visual” check would be the ultimate thing.
I think one could get to quantitative results : just mark up the inside of the fuel tanks with level markings when installing the cameras. These could then be read when looking at the images inside the cockpit.
Yes I understood you, just thought that kind of camera might do the job best. But I think I was exaggerating, for a permanent camera one could probably make do with a bit less image quality for a much lesser price. I like your idea very much. One problem might be that it doesn’t give quantitative but qualitative data, but pilots seem to love to crunch numbers…
@MedEwok, I’m sorry, I wasn’t clear. I meant permanently housed cameras inside the tanks, not an endoscopic examination as part of each pre-flight check.
I wonder if it’s possible simply to mount permanent cameras inside the tanks, with LED lighting. I can’t see why not.
This would basically require a medical-quality endoscope. These sell for the price of many smaller GA aeroplanes, which might be the only limiting factor next to the ridiculously conservative mindset of the GA industry.
Note that like with things labeled “aviation grade”, “medical grade” products are often sold with a Base price * X price multiplier, where X is by default a number that seems to have been created by throwing g at least two or more D6 dices…
