Well a historical linguist compares languages at several levels. You start out looking for basic vocabulary. All languages of the world, natural languages at least, have words for eye and head and nose and ear and for sky and earth and for water, sand and for sibling, mother and father. They may not have words for uncle and aunt. It becomes much vaguer because in one culture an aunt is different when it comes from your father's side than from your mother's side. You don't include snow. Most people know what snow is but in the tropics you don't have it. So you look for notions that are totally comparable and that occur everywhere in the world. You take the hundred or two hundred most universal notions in a human life, those which you call the basic vocabulary. So you take basic vocabularies and languages that you think are related. You look for words that sound the same and basically you're not fooled by a hundred per cent identical words but you are really looking for words that sound the same.