Proofs that God can be seen. - David

David saw God:

1 Chronicles 21:16-17

 16 And David lifted up his eyes, and saw the angel of the LORD stand between the earth and the heaven, having a drawn sword in his hand stretched out over Jerusalem. Then David and the elders of Israel, who were clothed in sackcloth, fell upon their faces.

17 And David said unto God, Is it not I that commanded the people to be numbered? Even I it is that have sinned and done evil indeed; but as for these sheep, what have they done? Let thine hand, I pray thee, O LORD my God, be on me, and on my father's house; but not on thy people, that they should be plagued.

Joshua and all Israel saw Him:

Joshua 5:13-15

13 And it came to pass, when Joshua was by Jericho, that he lifted up his eyes and looked, and, behold, there stood a man over against him with his sword drawn in his hand: and Joshua went unto him, and said unto him, Art thou for us, or for our adversaries?

14 And he said, nay; but as captain of the host of the LORD am I now come. And Joshua fell on his face to the earth, and did worship, and said unto him, what saith my lord unto his servant?

15 And the captain of the LORD'S host said unto Joshua, Loose thy shoe from off thy foot; for the place whereon thou standest is holy. And Joshua did so.