The typical test for reading glasses is a diopter test. This tests determines the corrective strength needed to read small text at a normal, comfortable distance without straining.
Diopters are the units which measure the corrective strength of reading glasses. Corrections are measured in increments of 0.25. A larger number indicates a strong correction.
The diopter test, which can be found for free online, shows the different corrective strengths on the left. A sentence is printed beside each strength. The size of the font of the sentence corresponds with the amount of correction needed in order to read it comfortably.