A sign - If the Angel ate of Gideon's present it would be a conclusive proof of the reality of the vision. (Compare John 21:9-13; Luke 24:37-43; Acts 10:41.) It would also be a token of God's goodwill to Gideon. Compare Genesis 18:3.

Show me a sign - Work a miracle, that I may know that thou hast wisdom and power sufficient to authorize and quality me for the work.

And he said unto him, If now I have found grace in thy sight, then shew me (g) a sign that thou talkest with me.
(g) So that we see how the flesh is the enemy of God's calling, which cannot be persuaded without signs.

And he said unto him, if now I have found grace in thy sight, &c. Or seeing he had, as appeared by his salutation of him as a man of might, by the work he gave him a commission to do, and by the promise of assistance and success:
then show me a sign that thou talkest with me; in the name of God, as a messenger sent by him, whether an angel or a man; for who he was as yet Gideon was not clear in it, and that what he had said was truth, and would be certainly fulfilled; and which Gideon might desire, not so much, or at least not only for his own sake, and the confirmation of his faith, for which he is renowned, as that he might be able to satisfy others that he had a commission from God, by a messenger of his, to attempt the deliverance of Israel.

That thou - That it is thou, an angel or messenger sent from God, that appears to me, and discourseth with me. Or, a sign of that which thou talkest with me; that is, that thou wilt by me smite the Midianites.