At the moment ROAR specifies minimum terminal resistance only for 25.5 motors: 101 milliohms at 25C. Reasonable numbers for the other winds would be 64.9 milliohms for 21.5, 42 milliohms for 17.5, and 25 milliohms for 13.5. All of these are also at 25C.

You'll need an accurate milliohmmeter, which isn't cheap, and a thermometer. If speed of measurement is not a priority, here's a less expensive way to do the test:
http://www.rctech.net/forum/radio-el...l#post14329133