[dr_orpheus]

No spacecraft has a "fuel gauge" in the traditional sense. You don't have gravity to for a float to work like any fuel gauge in a car or airplane. Fuel is driven by surface tension and pressure rather than any buoyancy/gravity forces. These leads to the development of diaphragm type tanks where the pressurized bladder pushes fuel out of the tank or propellant management device (PMD) type tanks [0/1] that use the surface tension to guide pressurant-free fuel to the thrusters.

So determining how much fuel you have left is done by a combination of integrating how much time you had thrusters firing, coupled with what pressure the tanks/lines were at while the thrusters were firing. Errors in these measurements accumulate over time which is why there is a lot of effort in to determining how much fuel is left in a spacecraft. Especially critical for things like big comm birds in GEO where fuel can be limiting in operation and the more fuel you have the longer you can keep station and get revenue from the satellite. But you need to still be conservative enough to have enough fuel to get out of the GEO belt for decommissioning your satellite.

[0] https://en.wikipedia.org/wiki/Propellant_management_device [1] https://www.northropgrumman.com/space/pmd-tanks/

[RugnirViking]

I'm sure we'll get there eventually. But there is a fun idea in there somewhere about a future where the only need for manned spaceflight is so we can get pinged by the computer once every few days and go and hug/wrap a tape measure around the fuel bladder