My grill literally has a gauge that works off the weight of the tank. I think it’s prolly accurate enough. Propane tanks are mass produced to fairly rigorous specs, as they have to contain pressure for safety.

It’s the most accurate way to measure fuel in a tank. Pressure gauges are much less useful.

It doesn’t need to be super accurate, just enough to tell them if they’ll need to get a new one soon or not. They could probably just pick it up and guess, but at least this gives them a number to track.

They usually come with a tare (empty) weight stamped on them. Total weight minus the weight = gas weight