Well, that's fine for electric powered planes, but I think he is talking about how to know when the GLOW plane is running low on fuel, and I believe Kitbuild said it... simply by flying and checking the amount of fuel left after, say, 10 minutes of flying, landing, then de-fueling. Then you can judge how much fuel a typical 10 minute flight with GLOW will be and fly accordingly.

For the glow powered planes, the receiver battery can be monitored, as pointed out by MetalicJunkie, using the Voltwatch 2 that his link points to. But, it's always a good idea to keep a good quality "loaded" voltmeter on hand.. one that puts a load on the battery pack. The voltwatch 2 works as though it has a load because it is activated when you power up your glow receiver and the load it monitors is what the battery pack is currently "seeing". Simply wiggle the sticks and watch the LED's. If it goes flickers to the red, either you have a bind in the linkage or it's time to recharge.

CGr.