That would have trouble passing anti-discrimination requirements: disability (no hands), medical (bandanna covering cancer treatment hair loss), religious (burka, rasta, yarmulke, sheitel), racist (cornrow).
And trouble with: dreadlocks (can’t run fingers through), bald headed guys (as mentioned by sibling comment), and people with hairdo’s (coiffure, hairspray, topknots, plaits etcetera).
It doesn't need to be literally their hand through their hair, it just needs to be some action which is easy to perform but complicated to photo-realistically simulate in real time from an arbitrary starting condition. Have them tug on their clothes to see how the fabric moves, have them or a caretaker turn a nearby light on and off such that their illumination changes, etc.
That would have trouble passing anti-discrimination requirements: disability (no hands), medical (bandanna covering cancer treatment hair loss), religious (burka, rasta, yarmulke, sheitel), racist (cornrow).
And trouble with: dreadlocks (can’t run fingers through), bald headed guys (as mentioned by sibling comment), and people with hairdo’s (coiffure, hairspray, topknots, plaits etcetera).