This test can be easily performed on any individual who is able to maintain fixation on a target. One eye is occluded and the opposite, uncovered eye, is observed. If there is no movement of the uncovered eye then it was aligned and fixating on the target, eg Snellen letter. However, if the uncovered eye moves to pick up fixation then there was a deviation (tropia) present.
An inward movement means the eye was exotropic and an outward movement to take up fixation implies the eye was previously turned in (esotropia). If no tropia is detected then the other eye is occluded and the test repeated.