How do i find out what refractive error is?

Ref error: Let your eye doctor measure you with an auto-refractor followed by manual refraction (dry and wet).

Ophthalmologist: If you need to find this out to get glasses, you will need a prescription from your ophthalmologist. If you already own a pair of glasses an optical shop can tell you the numbers. The reading takes specialized instrumentation and you cannot find this out from things found in the non-optical environment.