Yes, in order to determine the blood group, it is necessary to donate blood for analysis. Under laboratory conditions, your blood samples are mixed with tsoliclones (solutions of monoclonal antibodies to antigens A and B, which are located on the surface of erythrocytes and determine whether your blood belongs to a particular group) and, by the presence / absence of an agglutination reaction, determine which group your blood belongs to. ) The method described by Matti Laine, in principle, can also work, but for this you need to be able to correctly interpret the results of the analysis: to know about the possibility of the existence of blood subgroups for the second and fourth groups and be able to distinguish between specific agglutination of erythrocytes from nonspecific =)
You can also ask your mom. This is how I recognized my blood type. Apparently, they are given this information in the hospital at birth.
It is also possible, if you are healthy, to become a blood donor - then you will be tested for free and quickly not only for blood type, but also for the main dangerous diseases.
Take blood samples from people whose group is reliably known and mix with samples of your own. The group number of the drop where clotting will not occur will be the number of your blood group. Roughly speaking, this is the basis of a medical test for a blood group. A standard serum is applied on a special plastic plate (plate) with depressions (holes), and then a drop of your blood is added to each plate. Where there is no agglutination reaction (gluing of red blood cells) - that hole is your blood type.