Try focusing directly near the corner at something far with small details, you can hold your elbows on a surface or something…I did try without the tripod - as well as doing it immediately after turning the camera / moving the tripod around before setting it down and immediately taking pictures.
The issue is that I am not so steady and it was darkish in the room I was testing in so hand-holding at 1/10th wasn't really consistent for me even with IS working 'normal'. Even with clear pictures - there was no way I couldn't be sure any twist / movement of the camera wasn't me moving.
I'll need to mess around with it some more when I have more light to work with. I was stopped down so I could make sure the edges would be reasonably sharp making it easier for me to detect issues.
I do also have an RF 24-70mm f2.8L - I can test with that one too. I also have several EF lenses. Would you think it better to test wide open or zoomed in? I can certainly do both but would like to focus on the most likely opportunity to catch issues.