I have found that it is very simple. If on late Sunday afternoon you start to get a feeling of dread in your gut because you have to go to work on Monday morning you are not happy and there are better things that you could be spending your life doing.
Yes. Then pesky things like a mortgage and bills get in the way and you have to decide: Shall I live or shall I live?
On another note, I hated elementary school, middle school, high school, and college. I couldn't wait to graduate all of them, and have not found myself pining for the "good old days" just yet. I much prefer the "real world," where there is no such thing as homework in spite of all my teachers and everyone else cramming the idea into my head that I'll be taking my work home with me. Not yet anyway, thank god.