My supposition is that their request to send spies out into the land was in reality more a statement of unbelief rather than believing God's word. God knew all about the land and the people who lived there. The people didn't need to send in the spies, but only to trust God's word that He was giving them the land and he'd be with them in the process. They didn't need to look over the land and "check it out"....to verify what God had already told them. The resulting report from the spies scared them and they doubted God's ability to give it to them . Considering how God had recently brought them out from Egypt and cared for their every need thus far, they still couldn't trust God that He had the very best in store for them. They worried that their children would suffer. Yet it was their very children who forty years later who would inherit the land since everyone over the age of 20 died off in their wandering in the wilderness due to their unbelief except for Joshua and Caleb .
We have only to look at the how the walls of Jericho fell down to see that God seems to always give us a task which can't be humanly done in order for us to realize that it is God alone who does it and deserves the praise. If God gives you something to do, just do it! Never mind about spying it out or using Gideon's fleece. Give yourself to God and let Him work through you. If you do it alone, you will fail. If God does it through you, it will succeed, but not because of anything you did. Could I boast and say I have always done things the right way? Absolutely not! But I have learned a lot over the years. Aren't you glad that He is a God of second, third and so forth chances?