Some mowers and small tractors have a gauge built in the gas cap.
It works by a cork float that rides up a spiral mechanism that makes the indicator
show Empty to Full.
Trouble is, finding the right diameter cap to fit your tank,
also, clearance is needed to insert the gizmo. The way to go is what everyone
else has mentioned, a petcock (that's the real name of the fuel valve,no kidding) that has a reserve setting, and like Ree said, knock on the tank, when it's full and empty. If your speedo works, calculate and watch your mileage on a tankfull.
My problem is sometimes I forget to turn the damn valve on, boy that's embarrassing if your riding with a group. I think that's happened to everybody at least once.