You get cranky when you don't have time to write.
You get an idea and have to jot it down no matter where you are--on a napkin at a restaurant, on a receipt in the grocery parking lot, on your two-year old's forehead...
Your best friend tells you her deepest, darkest secret, and you think, "That would make a great story (with name changes, of course.)"
You have to periodically shake crumbs from your keyboard, since you eat your meals in front of the computer.
You wake up at 2 am and grab the laptop when you get an idea for your plot, and you ignore the grumbling spouse beside you.
You write a love scene and want to try it out on said spouse. :)
Now, it's your turn.You know you're a writer if...