A DNA test is the only accurate way of determining if a child is yours. In some cases, a child may have a blood type that is genetically incompatible with the father's blood type, so that it is impossible that they are his child. But even if someone isn't the father, they may still have a blood type compatible with the child, so this isn't a foolproof method. Blood types can never confirm that someone is the father, but they can sometimes exclude someone from being the father.