If you know the Ohm range, you could check it with a multimeter, but at the very least, start checking for a signal coming into the gauge. A grounded out gauge or sender will also peg the needle. So start by checking the electrical connections at both ends of the circuit (gauge and fuel sender). If all checks out, then remove the float assembly and inspect it for damage or binding. You should be able to power it up and move the float up and down while checking the readings coming off the signal wire to see the Ohm variations.