(All of the following is for small planes. Big guys may differ.) Yes, there's a drain at every low point in the fuel system. Before each flight, you sample fro

(All of the following is for small planes. Big guys may differ.) Yes, there's a drain at every low point in the fuel system. Before each flight, you sample from each drain, discard any water (and get concerned if there's more than a drop or two), confirm that the fuel is the color you expect (different types and grades of fuels are dyed differently), and then (especially if you're in California) return the sampled fuel into the tank, at which point you either visually check the fuel level or, if it's not at an an easily recognizable fill level (many planes have both "full" and "at the tabs", fuel up to a metal protrusion that marks a known quantity), "stick" it with a calibrated stick that gives you a pretty good estimate. In cruise, you're at a known power level, and you know your fuel burn at that power level pretty accurately (± 5%, say). In addition, many fuel systems have a fuel flow meter that is accurate enough to integrate to get fuel burn that way, with comparable accuracy. And of course you have the fuel senders in the tank itself which feed one fuel gauge per tank.