You could certainly watch the fuel gauge decreasing in an F-4. At low level and full reheat, the quoted fuel consumption is 2000 lbs/minute, which is roughly equivalent to 4 (imperial) gallons/second, say 4.5 US gallons/second!
On another note and in keeping with this thread, I was lucky enough to bag a ride in a German Navy F-104 out of Eggebek in April 83. We did a low level jaunt around the altic and flew close enough to Peenemunde to be able to see the MiGs on the airfield. I had expected the 104 to be twitchy in roll, but it was rock-steady and terrific for close formation. On the other hand, I seem to remember final approach speed on the day (2 wing-tip tanks) was 195 kts! To aid the discussion on endurance, my logbook shows a sortie length of 1 hr 20 mins, all at low level.