Your easiest bet is to find a teardown video and pause when they have the case off.
The coil is the coil shaped bit in the black plastic on the left.
Technically, you could get one of those meters that measures the power draw of a device, hook it to a wireless charging puck and slide it around. The power draw will be higher the more closely aligned the two coils are.
Going that far is going to be really fiddly and probably not worth the trouble though.