Get a BENCH POWER supply, (not a charger) , OR a 12 Volt Coleman Lantern Battery, (DO NOT USE THE CAR BATTERY..If you get a short you can injure yourself or start fires or explode a battery)
Locate the Power , Ground, and Sender sides of the gauge..Most are marked "S" , "G" and "I".... I is power to your source..
"G" Is ground.
"S" is sender..
Run a ground wire from your supply or battery, to your SENDER BODY, and the sender signal wire back to the "S" terminal..
Power your system up, (advise a 3 amp fuse) , and note reading on the gauge, It should be "E"...Than raise the float slowly and watch the gauge...1/4 travel should be 1/4 index increment..1/2 travel should be about 1/2 index increment..and so on until "F"..
If it full scales or does not respond with little movement, try one of the other's...If it seems to track OK, with , say an error in the middle or 1/2 tank it may be OK, with a linearity problem..not a whole lot you can do about that..except try and bend the float to make it a bit closer OR just make a note of the non~linerity..and keep it with the system when you install it.