1. When the words come easily and I don't have to struggle for them
2. When the first draft is complete and for a few moments I have this feeling like 'I am freaking brilliant.' It only goes away once I start editing, but it's nice while it lasts.
3. Getting a story accepted.
4. When someone tells me they like my writing and affirms my brilliance (hey, sometimes you have to toot your own horn.)