Do you mean how can you tell a person's blood type?
If so....
You can determine a person's blood type by a test called ABO typing which is a test that takes your blood sample which is than mixed with antibodies against type A and B blood. Then, the sample is checked to see whether or not the blood cells stick together. If it does stick than it means the blood reacted with one of the antibodies which is than your blood type. If both clump up and stick together than that means. a person can have Type O blood which contains both types of antibodies.