What are the tests for refractive error?

Refraction. Refraction is the testing procedure which can determine refractive error. This is the process by which the glasses prescription is determined.
Sharpening vision. An auto-refractor machine finds the correction needed to make a clear image on the retina, and this is your "refractive error" that can be corrected with a prescription. People are now doing this with iphones for calculating sphere and cylinder.