Once the mount is horizontal in Alt Az mode and the black polar alignment cap removed, a spirit level can be put in its place and the position of the bubble noted. It does not need to be in the middle. The mount is then rotated in 90 degree steps and the position of the bubble noted at each step. If it stays in the same position the axis of rotation is vertical. The alternative is not to rotate the mount but rotate the spirit level but if the machining isn't perfect it can give the wrong result.
If the spirit level is aligned parallel with a pair of tripod legs land then the mount rotated by 180 degrees, both those legs can be adjusted equally in opposite directions to give no bubble position change and once that is done, the spirit level can be rotated by 90 degrees and the procedure repeated to adjust the remaining leg only for no bubble position change. Repeat the whole procedure as necessary.
Of course one could say why bother as it's unnecessary, but some people might find this useful.
If the spirit level is aligned parallel with a pair of tripod legs land then the mount rotated by 180 degrees, both those legs can be adjusted equally in opposite directions to give no bubble position change and once that is done, the spirit level can be rotated by 90 degrees and the procedure repeated to adjust the remaining leg only for no bubble position change. Repeat the whole procedure as necessary.
Of course one could say why bother as it's unnecessary, but some people might find this useful.