It can be worked out by testing how they respond to different colors and by examination of the cone cells in their retinas. Dogs also tend to be near-sighted relative to humans. A dog’s visual world is rather drab and somewhat blurry compared to ours. On the other hand, they have a wider field of view, see significantly better at night, and have superior motion detection. It’s hypothesized that our sharper, clearer vision emerged in tandem with bipedality, because when your eyes are several feet above the ground, seeing distant objects becomes both practical and valuable. (Same with birds.) Animals whose heads are close to the ground, on the other hand, usually have a much better sense of smell than we do. Our sense of smell stinks.
Dogs aren’t actually color blind, but they have only two types of color receptors instead of the three we have, so the range of colors they can see is much more restricted. They can see shades of blue and yellow but not red, green, or orange, for example.
Turning into a Magic School Bus adventure.