Apparently, you’re supposed to see a circle as your mouse when using responsive mode. I have never seen that, even using several different machines, so the feature seems broken.
Updated for v72:
Older
However, right after posting this question, I finally stumbled upon a solution. In version 50, the solution is now to press the three dots ⋮ to the right in responsive mode, and select Show device type. This gives you a new dropdown next to the sizes etc., in which Desktop or Mobile (no touch) can be selected to use a normal mouse pointer (disable touch emulation) while using responsive mode.
