How do people know how animals and insects see?

Whether it is black and white, seeing 6 of things or anything else, how could we possibly know.

In the past, people used experiments to determine the nature of a creature's sight. nowadays, it's as simple as hooking up a critter's optic nerve to a monitor, then we can see what they see.

I think there's been animal studies for years to see how animals can see. Like I think there was a study with color to see if dogs see color, but it turned out they were color blind. Obviously the dogs can't talk, so they'd find out that they can't see by doing the type of test that are administered to humans to see if they're colorblind. A colorblind person can't see the number 67 in this specialized dotted color pattern, and they did something like that with dogs (obviously not the numbers thingy, but somehow trained the dogs to recoginze a pic, but then when they changed the color of the pic, the dogs wouldn't react to the photo).

Thank you, that is my question. People say they do experiments, but how?