Hi Steve
You just use a cheap laser pointer you can get anywhere for a few dollars.
I find it a frustrating test to use as the surface has to be extremely clean for it to work properly. I saw someone with experience on cloudy nights recommend this recently:
https://www.amazon.com.au/dp/B00LAX5...302_E_DDE_dt_1
I bought one. I haven’t used it on a mirror yet but it has impressive resolution for something so cheap. Should be good for checking pit sizes look even.
Rod