How do optometrists know if a baby needs glasses?

Retinoscopy. Using a retinoscope and handheld lenses, they can accurately determine the baby's refraction and then determine if it is appropriate to prescribe glasses. This is also used in the mentally handicapped who can't participate in the refraction process.
Glasses. Eye doctors can use an instrument called a retinoscope to determine if the baby is near-sighted, far-sighted or has astigmatism that requires treatment with eyeglasses.