Another test you can use is: always produce. For example, if you have a day job you don't take seriously because you plan to be a novelist, are you producing? Are you writing pages of fiction, however bad? As long as you're producing, you'll know you're not merely using the hazy vision of the grand novel you plan to write one day as an opiate. The... See more
I’m hesitant to cite Ernest Hemingway (because, Hemingway), but in A Moveable Feast he wrote about knowing when to stop writing each day so that his subconscious could take over, and he would refill, like a well. For me, refilling often means biking around Central Park.