Whether you have one wing tank or two, you need to know how much fuel you are carrying. Avid fiberglass wing tanks lend themselves to direct reading of fuel level through the butt rib via a "fuel window". This is often calibrated for in-flight as well as on-the-ground fuel levels. Additional light filtering through the top of the tank will make the fuel easier to see - leave a small area uncovered and unpainted.
Examples of fuel tank marking
Clear, simple, easy to read.
View from inside the cockpit.
These markings are more difficult to read.