I recommend reading "Will It Fly" by Pat Flynn and "The 4-Hour Work Week" by Tim Ferriss.

Go ahead and run a Google Adwords Campaign to a landing page with the option to purchase something related to your topic. If you get a good conversion rate of people who click the Buy button, then it is a winner.

Ideally, you are looking for a topic that is a huge need for your target audience. A friend of mine always says that he is looking for at least an 8 out of 10 on the BIG Problem scale.