What is the easiest way to identify what leg of a 220V panel a 110v outlet is on?
This is assuming North American 120:120 wiring. 

Suppose you plug an extension cord into the outlet, then run it back to the spot in the house. Also suppose you have a voltmeter rated for safe mains operation and know how to use it (if that's not true, get assistance from someone (friend, neighbor, licensed electrician, whatever) who does know how to do these things safely. 

Identify the 'hot' wire on each pair (should be 120VAC relative to earth). Measure between the two hot wires. If they're on the same leg, the voltage will be very low. If they are on opposite legs, it will be more like 240VAC.