1.) create a free website at http://wordpress.com and make 2 pages. Page 1 describes your idea, and includes a link to Buy Now! Page 2 says "sorry" out of stock and asks for an email. Now send everyone you know to the page... And see how many you 'sell'.
2.) Setup some interviews with people who should really want your product/service. Ask them about the idea and ask how much they might pay for that... judge the amount as validation!
3.) Go to a trade show and try to hand out a flier about your idea... Judge people's reaction and ask them what they think.
4.) Search the web for similar products and especially for targey customer types actually complaining about the problem your idea solves. Hear nothing/find nothing.... Probably not good for your idea.
5.) Make a mockup of your idea and stand at a corner where target customers might walk by. Ask people if they would like to learn and what they would pay for that.
The key idea is: will your target customer pay for the idea?
Go out and see BEFORE you build it.
Thats lean startup.