When I write something that would have made me laugh as a 10-year-old, or would have scared me or would have excited me, I know I'm onto something.
When I was young, I kept a diary for about 10 years and I had to write in it every day. Even on days when nothing seemed to happen, I made myself think of something to put in it.
Getting that first draft out is a horribly hard grind, but that (perversely) is where the joy of it lies.