There are various tests, but the most commonly used is the Ishihara test.

In this test, the optician shows the patient a number of plates which are printed with different coloured dots of differing brightness, density and size, arranged so that the dots of similar colour form a figure (for example a number) among a background of dots in another colour.

If a person has defective colour vision they'll be unable to identify some or all of the figures hidden in the dots depending upon what type of colour blindness they suffer from.