Do you mean literally? In that case, through the Scriptures, through oral tradition, through the Magisterium of the Church, through the Written and Oral Law, and so on.
For theists, there is faith that G-d, in whichever form or manifestation one believes, is the epitome and essence of morality and so whatever He commands is, by definition, a true moral value. Judaism, however, steps outside the Euthyphro dilemma: does G-d command morality because it is good or is it good because G-d commands it? It tends toward the latter; but the question itself is irrelevant according to Jewish thought.
