How do you know it is "xx% capacity"?

If you want to know degradation, make sure that the battery is at room temperature, then:

1) Fully charge vehicle (extended) with the J1772 port (not CHAdeMO)
2) Battery must be near room temperature
3) Turn off cabin climate control with "OFF" button
4) Disconnect the 12 volt battery negative cables and leave off for several hours minimum (overnight is ideal)

Next day:

5) Reconnect 12 volt battery
6) Hold foot on brake and press START button with key fob nearby
7) Energy gauge will show Full (16 illuminated segments) and "LO" on the GOM
8) Wait for navigation unit to complete its start up 
9) Press START with brake pedal depressed a second time
10) The displayed range is "RATED RANGE" at 3.5 miles per kWh

11) Divide this Rated Range by 146 to get usable battery capacity compared to new.

Example: 142 rated range / 146 = 97.2% battery capacity from new