Spindle Squaring

So when I use a surfacing bit on my MPCNC Primo it leaves pretty significant grooves on each pass over the material, indicating that the bit is not parallel to the surface. What is the best way to ensure that the spindle is square to the machine?

I am using the Dewalt 660 for reference.

The search term you’re looking for is “tramming”

Tramming z axis

How I squared (trammed) my router

Tramming the primo


Thanks Dan. I like the spinning a dial indicator trick, I will have to try that.