Simple, just fly until it runs out of fuel, then you'll know its empty Sorry, just couldn't resist that one.

You need to time a full throttle engine run on the ground and see how long it runs. Use a timer set at 1 or 2 minutes less than the amount of time it took to run out of fuel. Since you should fly at less than full throttle, you should have plenty of fuel left for a couple of balked landings.

For balancing a prop, I use the Top Flite Power Point Balancer
http://www2.towerhobbies.com/cgi-bin...p?&I=LXHY61&P=

Just make sure you use it in a draft free environment as even a small draft can cause a prop to rotate.

Hogflyer