How to test unknown fuel gauge.

Only way I know (this IS installed / working in a car right?) is to carry a gallon of "spare" in a safe manner, drain as much as you can (siphon, disconnect pump, drain into can) until you get the gauge "down" and then drive it till it drops. Heck you can just drive it round and round and round and rou........the block as long as the neighbors don't get upset LOL