You hook the Watt meter between the battery and the speed controller, brushed or brushless.
If you want good service life from your lipo batteries you will choose a battery that has ample capacity to allow you to stay within the continuous "C" rating of the battery even at full throttle. The accuracy of "C" ratings on lipos is all over the map, this particularly applies to lipos bought on the cheap from off shore vendors, if you pay a little more for name brand packs sold by distributors that have a good reputation to maintain you will do much better.
Good luck,
Pete