Fuel gauges have to be accurate for a flight operated under part 121 rules the FAA applies to scheduled airlines. If a gauge is inaccurate it has to be written up and deferred in the logbook. In that case the aircraft has to be sticked. It’s not a simple dipstick. They are magnasticks that come out the bottom of the tanks. There can be 8 or more sticks that must be pulled and then the pitch and roll of the aircraft read off other gauges. The stick readings and pitch and roll produce a quantity. This has to be crosschecked with the arrival fuel and quantity added. If they match you can depart. Enroute quantity is also monitored by a totalizer using fuel burn. It must work if a gauge is inop.