You probably won't be able to see the crack unless you see exhaust vapor getting thru it. One thing you can do to search for a crack or leak is to run a SMALL amount of liquid water into your intake manifold and watch for the steam to come thru the opening.
*edit* sorry, I had jsut gotten done typin up something on how you might have a broken stud, then I saw that you already checked for that. posted edited
