There are tests that can be used on infants and preschool children who cannot read. If you have any concerns about your child's vision, you should find a local optician who will test their vision.
However, if you have no concerns it's perhaps better to wait until your child is a little older - say around three and a half years - before taking them for a full eye examination.
Even if your child's vision seems good, it's worth taking them for a full sight test at a qualified optician when they reach school age, just to make sure that there aren't any issues with their vision that are likely to affect their early school years.
Find out more about Children's Eyecare