WHichever way it'll hang. If the actual center of mass is above the spot where you choose to suspend the airplane, then it is highly unlikely you will be able to determine if the plane balances because the plane will teeter over. If the center of mass is below the point where the plane is suspended, then the airplane will hang there.

Rule of thumb: A high wing plane is balanced upright, and a low-winged plane is balanced upside down.

You can hang the plane by a wingtip; straight down from where the plane is suspended is the actual center of mass. if you hang it from where it is >supposed< to balance, and the fuselage is level, then you got it.