As shown in the attached illustrations, Earhart could check the indicated fuel level in each tank. How accurate the indicated level was is a different question. The only way to be sure you have used all of the available fuel in any particular tank is to run it dry. When you know it's getting low, keep a sharp eye on the fuel flow meter. When the pressure drops, switch to a different tank and hit the boost pump. If deftly done, the engine won't miss a beat.
My guess is that, from experience during the world flight, Earhart had a pretty good feel for how accurate the fuel quantity gauges were.