You can convert your coaster brakes to not be brakes if it does prove to be the problem. Coaster brakes are terrible on motorized bikes, they burn out quick. That's one way, but Stan just told you how to eliminate it - remove the motor's drive chain and see how your wheel moves.
I know with my WCC bicycle, it's got dead coaster brakes, and it was basically squeaking until it froze, and now it won't move an inch.
Long shot, but maybe your master link is catching? If the c-clip is on the outside, try putting it on the inside. Also, how much chain tension is there? Check for slack, roll it forward until it stops, and check for slack again while you keep it slightly pushing forward. If the tension is different between those two points, then it's driven sprocket alignment.
For adjusting the clutch, don't rely on the bar - you want to remove your crank case cover and adjust the clutch plate.