How did Gideon know it was the Angel of the Lord that was talking to him?

ANSWER

he caused fire to miraculously consume the sacrifice

Judges 6 :

20 The angel of God said to him, “Take the meat and the unleavened bread, place them on this rock, and pour out the broth.” And Gideon did so. 21 Then the angel of the LORD touched the meat and the unleavened bread with the tip of the staff that was in his hand. Fire flared from the rock, consuming the meat and the bread. And the angel of the LORD disappeared. 22 When Gideon realized that it was the angel of the LORD, he exclaimed, “Alas, Sovereign LORD! I have seen the angel of the LORD face to face!” 23 But the LORD said to him, “Peace! Do not be afraid. You are not going to die.”
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