How to identify a stepper motor and AC servo motor? (by its appearance)

Hi guys,

I'm working on the motors during this semester, and I have an assignment like this:

How to identify whether a motor is a stepper or AC servo motor w/o open the inner parts (recognize it through its shaft, cover, fan, base... only), because they look almost the same for newbies?

From what I have found out so far, the only difference is the diameter of the shaft, and stepper motor doesn't have an encoder while AC servo always has one. I didn't find any difference in cover, base or fan (stepper has no cooling fan although it dissipates a lot of heat?).

Thanks.