Depends on the idea and how you plan to market it. Some you really can't test. If you plan on advertising, easy. Just run a very small ad campaign and check conversions.<p>Create a landing page that garners interest and asks for the users email and lets them know when the product is finished.<p><a href="http://www.mailboxapp.com" rel="nofollow">http://www.mailboxapp.com</a> was a recent example that started testing the market with no product.