A newborn brain is learning for the first time to move the eyeball, focus the lens, then keep the eyeball directed towards an object, and even then, needs to process the variation in information between two eyeballs. I'm not in this field but holy moly. Remember that newborn human brains are still in development after the womb, and it's not a high priority of the brain to get eyesight adjusted immediately. It allows for more plasticity I assume for the brain to focus on other sensations first.

They insist newborns are not exposed to strong light or sunlight for this reason.

What's going in varies too much for a lemmy length response. There's dozens of possible causes.

Testing is fairly simple though. The doctors will check for observable factors. If the pupil responds to light, if the eyes track movement, if the eyes move together, or wander. They can check the retina for issues, as well as the rest of the anatomy of the eye.

As example, just shining a small, focused light is enough to check for pupil response, as well as blinking or other signs that the brain is getting signals from the eye, as well as whether or not the eye is working as it's supposed to.