There are several approaches, none perfect, but this one requires no equipment and makes sense to me:
1. Fully charge the battery on a reasonably warm day. Listen for the cooling system to be operating. This will assure that the battery is near the nominal 98 degree maximum operating temperature.
2. Open the windows, reset the trip meter, and set the heater to absolute maximum heating. This will cause the heater to draw about 6 kW.
3. The trip meter will progressively show the energy consumed. When the car give a "stop safely now" message (will take something like three hours) note the energy use. This is as good an estimate as I know of for the useful battery capacity.