What is God's will?

Here's a question that I've been thinking about more lately. We just sold our apartment building for more than expected and it seems like other things that we've been planning on doing are pretty much falling into place. How does one know that these things are coming about because they are the true will of God, instead of God giving us over to our desires because we're being close minded on it all? We've prayed about what He would have us to do with our lives now that we have another child on the way and Ben's seriously thinking of going back to school for his PhD. I'm just wondering - how do you know when something is the will of God or if you just want it so badly that you convince yourself this is what is best for you. I'm not saying that I think what we're doing is not God's will for our lives right now -- all things considered God has truely blessed us! I do see God's hand in it all, I just wanted to give an example. Maybe a better example (for myself) would be birth control. I've struggled with it a bit and Ben brought up a good point ... are the things that I'm having problems with caused from a lack of faith? Am I not trusting in God to supply all my needs, to only give me what I can handle? I really think that my problems are caused from a personal issue of not wanting all the responsibility. Although I do need to study the issue more, the arguments for are more convincing than the arguments against. So, I do think in this case my desires have overpowered what God's will is ... how often does this happen in my life and I don't see it?