Before facing Goliath, David said everyone would know that the battle belongs to whom?

Answer: The LORD

David said, "It is not by sword or spear that the LORD saves; for the battle is the LORD's, and He will give all of you into our hands."

“But David said to the Philistine, “You come against me with sword and spear and javelin, but I come against you in the name of the LORD of Hosts, the God of the armies of Israel, whom you have defied.”— 1 Samuel 17:45 (BSB)