Can do motor dyno w/o load?
 

I just got a motor rmp & kv checker the instructions says can check the motor's kv n rpm. Is it ok? Or with load better?

The new firmware from Castle allows to check the motor Kv. And it's a no load test according to the manual, for example result is 2500Kv for the 1515/1Y 2200Kv motor (2200Kv is the Kv measured at load or max efficiency).
So I would say without load.