KV command in terminal ( how to determine kv of a motor )

I'd like to determine the KV of a motor. what's the best way to do this?

Is that what the KV command in terminal does? how does it work? how do you use it? I've randomly got it to respond 0 kv or some other number but clearly I don't know how to use it.

I assumed realtime data spinning at a fixed duty cycle controlled by vesc-tool would be needed. but I still get 0kv except for when I randomly didn't. not sure what I did different.