Blue Sky: signature of living worlds

An alien being with vision approximately in the same wavelength range as ours could immediately infer from seeing the Earth's sky that the Earth is a living world.
 

The blue hue we see is caused by Rayleigh scattering (a quantum effect) that preferentially scatters shorter wavelength light, combined with two factors, one of which is peculiar to humans and the other of which is the key to the statement above. Humans have a deficiency of violet sensitive photoreceptors, which makes us see violet fields as bluer (or you could say redder, anyway less violet) than they really are. So the sky is "really" a bit more violet than we see it.

But just as importantly, molecular oxygen selectively absorbs light in the near violet, so that it's not just that the sky appears more blue, it actually is more blue. If it were pure nitrogen, it would be distinctly blue-violet in color. And so, a being from another living world who found himself magically transported to the surface of the Earth could infer at once that he might not be home, but he was standing on the surface of a living world whose atmosphere contained a substantial percentage of molecular oxygen.

Ain't the universe remarkable?