Agree with Hui Qun. Between the ages of 6 months to 3 yeaers, practice good eye habits like not sitting too close to the tv (I know of some babies that like to stand right in front of babies), or playing too much on the handphone.
If you notice your child blinking too much if he's looking at something, that's a sign to bring him to the eye doctor.
After 3 years, I believe in primary schools, they would have regular eye check ups to test their eyesights as well.