One way to see if air has lots of oxygen in it is to look for signs of lichen growing on tree limbs. (toddler-activities-at-home.com)
The jutting midface of Neandertals seems to have evolved to help get large volumes of air into an active body that needed lots of oxygen. (scientificamerican.com)
The result was a highly porous carbon nanotube electrode with lots of oxygens exposed on the surface, ready to bind with lithium. (sciencemag.org)