There are two usual symptoms to tell my boat fuel gauge is broken?
1. Fuel gauge does not read at all
Check the power with a multimeter at the gauge terminal. Test between the positive terminal on the fuel gauge and a good ground; the reading should be 12 volts. If it isn't, check the ignition circuit. If that reads 12 volts, turn off the ignition.
2. Fuel gauge stuck on empty/ on full/or with irregular readouts
One of the usual problems comes with the fuel sending unit, have it inspected by a professional technician to determine if you need to replace your fuel gauge sending unit.