How can you tell when someone is using the keyboard to turn as opposed to some other means?
When they use the keyboard, they will stand stiff and straight as a board and only shuffle their feet when they turn. Sometimes their bodies will kind of bounce up and down a little with each shuffling step. It's very awkward-looking and extremely slow.
People who turn with their mouse keep their feet planted and only twist (to a certain degree) their head/torso until the user lets go of the mouse button, at which time they pretty much take a single, more natural step to realign their bodies in the new direction. It looks much more the way normal people do when they turn.
If you continue to use the mouse to look 360 degrees, or after your neck/torso has reached its twisting "limit" (because you're not an owl), your feet will begin to shuffle but at a speed that keeps up with the pace at which you're moving your mouse.
It's sounds weird describing it but try it when you get the chance, you'll see what I mean.