Anything that needs rhythm (walking, playing music), balance (walking, moving from standing to sitting), spatial relationships (going through a doorway), and grip force calculation (picking up a plate) will show that I'm off.
Also having to make up nouns and adjectives because the real ones have disappeared behind the brick wall. Eg, reducing people to helpless giggles by describing the window in a tin whistle as the music hole.
I don't know whether facial expressions would work for me, as mine are very contrived due to autism. I had to learn to do them as deliberately as the way children learn to pronounce a sentence in another language.
Skin tone probably would, though, it's how colleagues at work used to know. It might only be because I have the fairest skin type humans are issued with and my colouring changes a lot during conversation. When I'm in PEM it apparently doesn't, it just looks grey.