Judges 6:36-40 (NLT)
Then Gideon said to God, "If you are truly going to use me to rescue Israel as you promised, prove it to me in this way. I will put some wool on the threshing floor tonight. If the fleece is wet with dew in the morning but the ground is dry, then I will know that you are going to help me rescue Israel as you promised." And it happened just that way. When Gideon got up the next morning, he squeezed the fleece and wrung out a whole bowlful of water.
Then Gideon said to God, "Please don't be angry with me, but let me make one more request. This time let the fleece remain dry while the ground around it is wet with dew." So that night God did as Gideon asked. The fleece was dry in the morning, but the ground was covered with dew.
Christians have used this story about the fleece as a method of determining the will of God. They will say something like "God, I want to do X, if you want me to do X, then I will take it as a sign of your approval if you will cause it to stop raining tonight." This is a dangerous misunderstanding of this text. Certainly Gideon lacked faith and it is wonderful that God was gracious enough to condescend to Gideon's weakness. However, this request of Gideon's was not for guidance.
God had already told Gideon that he was to rescue Israel. Gideon was asking for reassurance that God would give him victory. In other words, Gideon was saying, "God, what you have asked me to do is going to take a really big miracle, please give me some assurance by first doing a small miracle as a sign?"
So how do you know the will of God? The model this text presents is this: God's will for you has already been revealed by His Word which in our case is the Bible. God can and I'm sure does answer such prayers for assurance today. One point however, Gideon's request was for God to do something that could only be explained as a supernatural miracle and not as just a natural coincidence.