This is how:
- set up your mount
- point telescope or camera towards Polaris
- take long (30s?) exposure of which the first 10s are static and the last 20s rotating the RA axis for 180 degrees
This results in an image that both show a star pattern and a set of circles.
By “plate solving” (finding the celestial coordinates) of the star pattern and drawing a RA, DEC grid you can see how much your polar alignment is off. Perfect polar alignment will result in concentric star trails and RA DEC grid.
Plate solving can be done with astrometry.net.
