A legal grandparent DNA test can help you find out whether a child is a biological grandchild of the alleged grandparent(s). The grandparent testing is usually performed when a child’s alleged father is not available for paternity testing and the child’s grandparent(s) doubt whether the child is their grandchild. A legal grandparent test can be used to establish the parenthood of an individual for a court case such as child support, social security, or child custody.