One of the better methods I found to test AF involved using different colors. You could create an image at your monitors resolution with 10 or so thin bars, each with different colors from the visible spectrum and then one by one focus on each bar - you might notice, some colors affect the AF more than others. Red was worst in my case.
Here's the same thing basically but performed in a slightly different way: The colors are illuminating from my monitor:
https://www.pentaxforums.com/forums/general-technical-troubleshooting/191214-...ml#post2013534