In Judges 6, God calls young Gideon to rise as a deliverer of Israel. The angel of the Lord appears to Gideon (6:11-12), and commissions him to fight and win (6:14-16). Gideon asks for a miraculous sign, as evidence that it truly is the Lord who speaks to him. God grants it (6:17-24).
That same night Gideon destroys his father's pagan altars, and offers two bulls unto the Lord. Surprisingly, his father Joash stands up for Gideon, and threatens anyone who would defend Baal, on the logical ground that Baal, if he's real, ought to be able to take care of himself (6:31).
Then a massive army of foreigners invade the land (6:33). God's Spirit comes upon Gideon, and he summons volunteers from the Abiezrite clan and three of the tribes of Israel. Then Gideon asks God for a second sign: he will put a wool fleece on the barn floor. If the fleece gets wet from the dew, but the ground all around is dry, then that will confirm the Lord's word. The Lord does it (6:38). Then, just to be extra-sure, Gideon asks the Lord to do the reverse. The Lord does that also (6:39-40).
Christians have sometimes sued this story as a reason for setting "fleeces" before the Lord. In other words, they propose that God do some distinct action that will confirm direction and future blessing from God. However, close consideration of the story tells us that what Christians present to God are seldom real fleeces; and nowhere in Scripture does the Lord tell us to do this.
First we see that the angel of the Lord had already appeared and audibly spoke to Gideon (6:11). This was not a case of inward impressions. Gideon saw a man-like figure. The man spoke words to him. The man touched the offering (meat, bread, and broth) with the tip of his staff, and a heavenly fire consumed it (6:20-21). One moment the broth, meat, and bread was there. the next moment, it was incinerated. So Gideon was not acting on inner impressions, or pious guesswork.
Second, Gideon was acting on the written word of God. This was early in God's plan, and Gideon would have only had the five books of Moses, and probably Joshua also. But they by themselves were sufficient to justify his actions that night. Naturally, the Scriptures he had wouldn't have said anything specific about him becoming a mighty deliverer of Israel. The Angel communicated that. But Gideon's forward actions were guided by the written word of God, not his own imagination.
Third, the fleece test was completely unnatural. It wasn't like some silly thing that young Christians, do, such as, "Lord, if you want me to marry that girl, have a robin fly past me today!" No, according to normal scientific laws, dew doesn't differentiate where it settles. Dew isn't going to soak the surrounding ground but leave a rug bone-dry. It won't soak the rug, but then leave the surrounding ground bone-dry. What Gideon asked for was impossible apart from divine manipulation.
Fourth, Gideon's request was spiritually suspect. His first request for a sign was reasonable. He wanted to make sure he wasn't being deceived (perhaps by an evil spirit), as he says in 6:17. He was testing the prophecy, as Moses taught (Deut. 18:20-22). But that first sign should have been enough. The second request came from fear. Even Gideon knew he might anger the Lord by asking (6:39). But God was gracious and patient.
There is a timeless principle in the fleece story, which is that the Lord verified His messengers with signs and wonders. The prophets could work wonders. Christ came with healing power. The apostles healed and worked wonders. Miraculous signs witness to the word of His grace (Acts 14:4). But Gideon's story doesn't authorize us to invent tests for God, especially lame, non-miraculous tests that could happen by coincidence.
Open doors are not a sure guide to God's will, either. Just because a door opens, doesn't mean God wants you to walk through it. Paul once had an open door to preach in Troas, but he traveled on to Macedonia to find his friend Titus, and there was nothing wrong with him doing that (2 Cor. 2:12-13). The New Testament tells us to test prophets, but it never tells us to set fleeces.
The fleece story shows us how patient the Lord is., especially with the faint-hearted. Gideon wanted to do the Lord's will, and in fact did do the Lord's will that first night. He wasn't a hypocrite or a coward. But his doubt of his own fitness (6:15, 36) led him to ask God for something very unusual.