I think this is one of those things that largely depends on the current state of your spiritual development. I believe we need to seek God first and all of these things will start making themselves obvious to us. Sometimes I wish it were still as easy to discover your purpose today as it was for some of those people in the Old Testament. I wish we had a prophet, a seer, or a high priest we could go to so he could communicate with God for us and get back to us with a correct answer immediately. On the other hand, I am glad we now have the ability to do that for ourselves without the middle man. It's just that doing so requires that we have that close relationship with God that they did.