Can you find some existing product/service in that niche, set up a SEO-friendly site, and test conversion rates right now? You don't even have to make a dime off the conversions for now.

That's one of the things Ed Dale's "30 Day Challenge" is good at teaching: how to "taste test" niche ideas. Unfortunately all the 2009 content is gone now because the 2010 starts next month. I wish they wouldn't do that. http://www.thirtydaychallenge.com/