I’m not 100% sure what you’re asking but I’ll try.

If you are asking in gereral; Most babies can focus quite well (like the lens in a camera) on an object but they see it in low resolution. Like they’re recording in 144p instead of HD. Their brains aren’t well developed but the resolution improves as they develop.

They can measure this resolution in a variety of ways. One way is an optokenetic drum, once the baby is a few months old. The eyes will follow a stripey pattern travelling in one direction. If the baby can’t see or resolve the pattern their eyes will not follow it. You can adjust the pattern (bigger or smaller stripes) until the baby can/can’t follow it. With toddlers you can show them two cards, one with a faint picture and one without a picture. Watch where their eyes go and go to the next set of cards.

If you asking how we know a specific baby can’t see, we measure the focusing ability of the babies eye with a special light and lenses. And how well the baby can see with and without glasses with the methods above. If the baby falls significantly outside normal values, we give the glasses and/or eyepatches. If they are slightly outside normal values we might re-examine them in a few months.