So Gideon has a three-way conversation with the angel and God, but he doesn't believe either of them. He demands a sign.
But first, he runs off to slaughter a goat. And then, guess what happened.
Then the angel of the LORD put forth the end of the staff ... and touched the flesh and the unleavened cakes; and there rose up fire out of the rock, and consumed the flesh and the unleavened cakes. Judges 6:19-21
Yep. The angel touched the bloody, dead goat and it burst into flames.
But it still didn't convince Gideon. He needed another sign to prove that God wasn't lying to him. So he put some wool on the ground and asked God to make it wet, while keeping the surrounding ground dry.
And Gideon said unto God ... Behold, I will put a fleece of wool in the floor; and if the dew be on the fleece only, and it be dry upon all the earth beside, then shall I know that thou wilt save Israel by mine hand, as thou hast said. Judges 6:36-37
And God passed that test, no sweat.
And it was so: for he rose up early on the morrow, and thrust the fleece together, and wringed the dew out of the fleece, a bowl full of water. Judges 6:38
Now you might think that would be enough proof for Gideon. But no. He's still not sure he can trust God, so he asks God to reverse the trick, and make the ground wet and the wool dry.
And Gideon said unto God, Let not thine anger be hot against me, and I will speak but this once: let me prove, I pray thee, but this once with the fleece; let it now be dry only upon the fleece, and upon all the ground let there be dew. Judges 6:39
And God did that trick, too!
So God passed all of Gideon's tests and Gideon and God got down to business. But first, they had to select some accomplices.