What signs and indications hint that the ghost is the dead king?

In Act One, Scene 1, Barnardo, Horatio, and Marcellus see the dead king's ghost and comment on its appearance. Barnardo comments, "In the same figure like the king that’s dead," and Horatio confirms that the ghost does indeed resemble the former king of Denmark (Shakespeare, 1.1.39). Horatio comments that the ghost is dressed for battle by mentioning its "warlike form." When the ghost disappears, the three men discuss its appearance and agree that the ghost resembles Hamlet's deceased father. Horatio also recalls that the ghost was wearing the same armor that King Hamlet wore when he fought against the king of Norway, and he mentions that the ghost was frowning like the king when he fought the Poles. In the next scene, Horatio tells Prince Hamlet that he believes he saw his father's ghost the previous night by saying, "The apparition comes. I knew your father. These hands are not more like" (Shakespeare, 1.2.211-212). After Horatio gives a brief description of the ghost, Prince Hamlet agrees to wait for the ghost to appear. Overall, the ghost looks similar to the deceased king, is wearing the same armor, and makes similar facial expressions.