Pretty simply, if it just won't get out of my head. If I can't stop thinking about it, then it usually means I need to get it onto the page.
I admit, I don't generally write creative nonfiction-- most everything ends up turning into a short story. I always think of the first line first, and then the rest of the piece just flows from there. If I think of a first line that I just can't resist, I know it's a story worth writing.
Hmm... That’s a tough question, because ideas can come from anywhere, really. To quote Neil Gaiman, “You get ideas from daydreaming. You get ideas from being bored. You get ideas all the time. The only difference between writers and other people is we notice when we’re doing it.”
Some of the time, a story comes to me when I have an idea for a character and their conflict, or a specific image or symbol. It starts with a small kernel and then I build around it until I feel confident enough to start writing. But sometimes my ideas come from really unexpected places (from things that I’ve read, or from the people that I see around me). So I think it helps to be receptive to any ideas that pop into your head.
Just a few months ago, I became inspired to write a story about a girl in Afghanistan who is learning how to skateboard, and the idea came to me because I read an article about a program called Skateistan, which is teaching young girls in Kabul how to skateboard. Normally, I don’t take much interest in writing sports fiction, but this stood out to me as something really different and something that I really wanted to see, as a writer. It was something completely random that I'd stumbled across by accident, but once the idea was stuck in my mind, I just couldn't chase it away. At the time, I didn’t have a specific character in mind yet, or even an idea for the storyline, but I knew that it was a story I needed to tell, because it kept coming back to haunt me. It wouldn’t leave me alone until I put words on the page.
So if you’re looking for ideas, I would suggest reading books for inspiration, or observing the world around you. Look for inspiration wherever you can find it, and if an idea just won’t go away, then chances are, it’s a story waiting to be written.
You’ll know that you have a good idea if it feels like poison ivy. You’ll need to scratch it, or else it will drive you crazy.