How would I know if my grandchild has a vision problem?

How old? Depends on how old your grandchild is. If newborn, check for an 'eye popping reflex' (he will close his eyes in bright light and open them in the dark). If a little older, check for 'fix and follow' (cover each eye and make sure he can track you as you move). If older, an eye doc can check with a vision chart.