Z axis tramming question

I have a bench top CNC mill where the head moves up and down as the Z axis. The table moves left and right as the X axis, and in and out as the Y axis. The head can also manually rotate around an axis parallel to the Y, but I’ve never touched this adjustment in the 15 or so years I’ve had the machine. Recently the head has started to appear very slightly off in relation to this axis. The error shows up in fly cutting along the X axis where the trailing edge of the cutter bites slightly more than the leading edge when the table moves left to right, suggesting the head needs to be rotated a fraction of a degree counterclockwise. I’d like to tram this error out of the Z axis but don't know how. I.e., how do I determine when the head is perpendicular to the table? Is there a special tool or gauge for this? I have a dial indicator that I use for tramming the vise in X and Y, but I’m not coming up with a reliable way to use it for this purpose. Thanks for any tips.

You need a dial indicator and some kind of an arm that you can chuck in the spindle (the photo is not mine, just found it in the interwebs):

https://www.model-engineer.co.uk/sit...036/720739.jpg

You rotate the arm and take the dial readings in four opposite points of the table (east-west-north-south). If the spindle is perpendicular to the table (and the table is flat), the readings will be the same in all 4 points.

Seems like what I’m looking for. Thanks!