This test is very similar to a test known as the Mallett fixation disparity test, which is used extensively by Optometrists (Mallett 1983).

 

The letters OXO are displayed with a red and green line on either side of the X. The user looks at the target through the red/green filters and is asked to say whether the red and green lines appear to be lined up with the centre of the X.

This measures how well the user can compensate for any muscle imbalances that may be present.