There must be a way to calculate the weight of an aircraft from the flight characteristics and from that get an idea of how much fuel the plane has. Maybe not 40 years ago...
Other than a nerd-sniping exercise I don't see the point of going down this rabbit hole, because on an aircraft with pax the pilot's job isn't to screw around playing Chuck Yeager and estimate fuel with test pilot techniques.
If they can't fly with confidence that they're safe, then the pilot's job is to put the fucking aircraft on the ground as soon as able.
Roughly, for each passenger there's an equivalent mass of fuel on the plane.
If you're working with thin margins that might not be enough, but if someone's shorted the craft half its fuel load, as was the case for AC143, I'd suspect this would be evident.