There are two principal methods that are used to determine if a horse is an actual descendent of a particular sire and dam.

1. Blood Typing: The same method used in blood typing humans is the same one used on horses. It includes using genetic blood group markers (the ABO and Rh system) and the electrophoretic method (using an electrical current to separate molecules according to their genetic makeup). The sire and dam of a certain blood type can only yield an offspring with a certain blood type. However, you must know the suspected parentage blood type to perform this test, which is more an elimination process than a confirmation. In other words, this process tells you who the horse's parents are not rather than who they are.
2. DNA Testing: Although not available to everyone and very costly, DNA testing is the new technology for parentage verification and is being used by more and more breed registries. It is a more accurate method of determination than blood typing. In addition, DNA testing doesn't require a blood sample. The test is commonly run from mane or tail hairs or a nasal swab, all of which are easy to acquire.