To some extent, I let "failure" weed things out for me.  Kind of like the story (about Reddit?) where the founders walked out of a law related class to go have waffles and realized they didn't really want to be lawyers.  If you just can't keep your hands off the new project and it takes on a life of its own, that is telling you something (though what it might be saying is that you just like doing the fun parts of the job and don't want to buckle down and get through the crappy parts of it -- so beware you don't misinterpret what it says).  On the other hand if it seems like a great idea but you would rather "go have waffles" (so to speak), then it probably needs to be someone else's baby (or not live at all).