When engine leans up, sound of engine would be more of high roaring-up rev-up. with lesser smoke coming out, and muffler will be very hot. When engine is rich, more smoke will come out of muffler, less stress noice will come from engine.

When fuel runs out, the engine will never be richer, it will be more leaner coz no fuel means no lubricant more friction, engine piston will be stressed. It is advised not to run-out fuel from engine during flight or when it is idle. To protect engine damage, you just need to put in at least 5 drops of After Run-Oil thru fuel line located at carb. Then use a starter to pump-in oil into engine, stop when oil is in engine. But, please bear in mind that, if you're flying weekly, then just skip this process. This process is when you'll be keeping your heli in storage for more than 3 months.