K-Moss It says there how TUV flicker free certified devices are being tested, https://www.tuv.com/world/en/eye-comfort.html
It does not go into detail, but I have realised there are stars also in addition to certification itself. It does not say how they test a flicker but you can see in image there of laboratory and this statement of stars - "The standards for 3/4/5-star ratings remain different. The higher the star rating, the more stringent the standards.". However, I was a bit sceptical of this certification, because many brands still keep it on OLED displays, when everyone is complaining about eye strain.