There are three tests used for Strabismus. The Hirschberg test using a penlight determines the type and degree of Strabismus. The Cross-over test reveal latent as well as manifest Strabismus. Finally the Cover-uncover test which reveal latent Strabismus. The tests are generally performed in that order.
The Hirschberg test
To determine the type and degree of Strabismus use a small penlight and direct it toward the eyes. The reflected point of light will reveal the type and degree of Strabismus. Normal eyes will have the light in the center of the pupils. Note that 1 mm displacement (Positive or negative angle Kappa) is considered .
The cross-over test
As the name suggest in this test an occluder is transferred from one eye to the other without interval. The longer the occluder is left over one eye before transferring it without an interval to the other eye, the more disruptive it is to fusion, tending to reveal the full extent of any Strabismus.
This test show any latent as well as manifest deviation.
The Cover-uncover test
In some cases latent Strabismus, heterophioria, is present, the eye will normally be kept straight by fusion. To discover the latent deviation cover one eye with some translucent material through which you can detect any movement of the eye.
If there is movement of the eye when the occluder is removed then there is latent Strabismus. The latent deviation will be revealed with either eye covered.