Yea, I'm aware of the polarity (also for generator) - this is how I define the "forward" and "reverse" direction, based on how it moves when I apply voltage with (+) to the red cable.
Thanks for answers, the conclusion would be that the safe choice is just to rotate it in the direction it was designed to rotate as a motor (+ to red). That is what I was thinking as well more or less although I had some argument for the other direction as well. The (weak and partial) argument relates to how torque is applied. I mean when engine has some load the torque is applied "one way" so to speak (one way of thinking about this is how the engine shaft or mount would bend if it would be maleable). In order to reproduce this "bend" when using it as a generator is to put some electrical load on the output and rotate the shaft precisely in the oposite direction. It would make a big difference if the mechanical part inside would be more complicated but I presume it's not, just normal rotor connected cu shaft).