My first thought on measuring the available fuel, after the gauges of course, would be a dipstick. A quick run through the Luke Field post-crash inventory shows nothing like that was carried then (doesn't mean it wasn't on the second flight), but ... if you REALLY want to know if there's gas left, how hard would it be to whittle something from the ample flotsam on the beach or along the jungle's edge? It would not tell you how many gallons were left, but it could, with careful calibration, tell you how much of what was left you were burning up every time you fired up the engine to transmit.
LTM, who finds no paint as interesting as dry paint,
Monty Fowler, TIGHAR No. 2189 ECSP