it will be easy if lcd/led matrix is rated for much higher refresh rate than actually used (very much higher). But usually it is close to actual refresh rate or even worse. In such case you cannot see pixel on/off event immediately, it will be very smooth and it will be hard to determine exact frame when pixel appears or disappears.
If video camera uses lossy compression format, it will be even more hard because compression will remove very small change of pixel brightness between frames.
If on screen trace refresh is keeping up with the LCD refresh, which on any modern scope should be 60-75Hz, then that would be great, perfect A rating, whatever you want to call it. What we are looking at with the siglent was probably sub 20 or 15 fps.
Could have two categories then to make it harder:
1) Single channel, no measurements on, no processing on.
2) All four channels on, 10 measurements on, something like that.
Though I would expect any good modern scope to not slow down noticeable with measurements or multiple traces?