When is a test cross used?

A test cross is used to determine if a group exhibiting a dominant trait is homozygous or heterozygous for that trait.

Test cross can be used to decide the genotype of the F1 generation. The test cross can be used to support the idea that the recurrence of the recessive character in the F2 generation is due to the heterozygous condition of the F1. The test used to prove whether any given pair of traits could be alleles (different characters).