If your child has a vision screen that is worse than 20/40 at the 3 to 5 year old wellness exam, you should see an ophthalmologist to find out whether they need glasses or not. If they are over the age of 6 and their vision is worse than 20/30, they need a formal vision screen as well.
Other things you can look for in your child are if they have an eye that tends to wander, especially when they get tired, because one eye can be worse than the other. They may not notice that, but the bad eye may actually wander, and that's what we call a lazy eye. So, if you notice a lazy eye, or if your child is squinting a lot, or needing to get really close to objects in order to see them, then you need to bring that up with your pediatrician.