Buy a circuit tracer, such as the following. (http://www.grainger.com/product/AMPROBE-Crct-Brkr-Finder-6NZF8?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/6NZF8_AS01?$smthumb$) You plug the transmitter into the outlet and the tracer tells you which wire/breaker in the box connects to that outlet. Find the ground wire that goes with that particular run of Romex and viola!
