1. For "general" fuel quantity warning use the "Fuel_Quantity_Ann" parameter. It will work with "test" button.
For separate Left/Right "Low Fuel" warning use the "Fuel_Level_..." parameters with assigned range (min=0, max=10 kgs for Cessna).
2. Normally, most of annunciators have related dataref in every plane flight model and work with assigned "Ann_Test" button.
But, if there is no "dedicated" dataref for some light (as for Fuel low above), you need to use the "Ann_Test_On" parameter, that indicates the test button is pressed. Just assign this parameter to the same LEDs that are used for low fuel annunciators.