Use a higher power eyepiece as you will get better diffraction rings both in and out of focus. Try racking inward past focus and outward past focus.
See
http://www.willbell.com/tm/tm5.htm for some example images of what you can expect to see. Note you are only collimating not evaluating the mirror surface quality so although steady air is best, you can do this in air that is shimmering and in light pollution.
Your aim is to have concentric rings at the highest power you can observe with as any error will be more visible at the higher power.