Probably look for display artifacts (pixel borders)? But a fixture that takes a good enough screen + enough distance to make the photographed pixels impercepti

Probably look for display artifacts (pixel borders)? But a fixture that takes a good enough screen + enough distance to make the photographed pixels imperceptible is likely just a medium hurdle for a motivated person. You probably can't fully avoid it but adding more sensors (depth) will make such a fixture quite a bit more expensive.