The easiest way to determine a person's blood type is to perform a blood test that checks for certain blood markers called antigens, according to WebMD. The test checks for two markers: ABO and Rh. The ABO test determines if the blood is type A, B, AB or O.
The Rh factor can be negative or positive, meaning there are eight possible blood types, according to the American Red Cross. Type O is the most common blood type followed by A and B. Type AB is the rarest type and accounts for approximately 1 percent of the population, according to the American Red Cross.