A capture card that the device's output passes through, hopefully unnoticed by both OS and display. As such cards are expensive, we haven't had many people try this thoroughly yet. It requires untouched recording.
I think the best choice would be an external capture card that is independent of the device whose output is going to be captured.
If we could find the cheapest external card that does what we need (plus we'd need to write some software to analyze the captured frames, but that part should be easy), we'd have a tool that people could actually afford and use to reveal temporal dithering.