fuel is displayed in a little rectangular screen just below the center-right EICAS screen (above the RCS knob in front of the throttle handle)
it shows three indicators - oxy, main, and asf
those are, respectively, your internal oxidizer supply (for rocket mode), you main fuel quantity (for turbines and ramcaster) and your Auxiliary Systems Fuel (ASF) reserves, which is used to power the APU (coming soon) and the OMS/RCS jets
note the blue dotted markings around the oxidizer gauge - and the way it points to a specific level on the main fuel gauge beside it
this is called the "full burnout point", or "oxidizer sync line" -- it marks the point where a fully loaded oxidizer tank will burn towards a synchronized depletion with the main fuel tanks
it is the optimal point to fire the main engines in rocket mode - if fuel is above that line, you'll have an amount to spare after oxidizer is gone (it's actually standard practice to leave some slack for powered landing and taxi) -- if you fire rockets with fuel below that point, you'll burn it out before your oxidizer is depleted - which is very inconvenient, since you can't use that for landing and it'll just weigh you down :hmm:
in WIP-3, the APU doesn't really do anything other than sound like it's running....
for WIP-4, well... it'll do more than that :thumbup: