The Snellen eye test is available at doctor's offices and optometrist's doctor's offices. There are also online resources to take the eye test, though people can only get an eyeglass prescription from an eye doctor. Online tests, such as the one available at VisionSource.com, can help to determine the approximate quality of vision.
The Snellen eye test is the main test available in regular doctor's offices and clinics. It includes different lines of text, where the letters get smaller as they go down the sheet. Some doctors have the individual start reading letters from the top line and work their way down. Other doctors ask the patient to find the smallest row of letters they can view easily and read them aloud. Either way, it determines the visual acuity of an individual based on how far down the chart she can get.
The placement of the chart is usually about 20 feet from where the patient stands to try to read the letters. There are also tumbling eye charts where a single letter is in a different formation, which is a test used for children who can't read letters out loud.
With the traditional Snellen eye chart, someone who can read the smallest row of letters at 20 feet has 20/20 vision. If an individual can only read the top row, or the largest letters, she probably has poor vision and needs glasses.