As for peeking into the tanks and seeing what's the we have
made calibrated dipsticks for every airplane we operate, with the zero
on the stick being the unusable-fuel level in the tanks as specified by
the POH, and full being FULL, which usually coincides with the tank
capacity given by the POH. These aircraft are used for training
commercial pilots who will be flying in Third-World
jungle/desert/mountain/sea operations, for outfits that demand an exact
fuel figure before every takeoff. Fuel mistakes in such environments
are usually fatal.
The calibrated stick isn't hard to make, but it surely is a pain
to get every drop out of the system, put the unusable fuel amount in
each tank, then add three or five gallons at a time to determine the
level for that amount. The airplane has to be fairly level, too,
without being rocked around by wind or moved while determining dipstick
calibrations.
The stick is handy for confirming accurate fuel flows after
landing from cross-countries and seeing what the real burn is at
various mixture settings. Cessna, for example, calculates their cruise
charts based on a pretty lean mixture, where few low-time pilots
operate.

Dan