This really depends on the idea that you want to test. Remember, a test is just a test, and not a complete depiction of how your actual business will do once it launches as that greatly depends not only on your product, but also on how your business is managed and what value you provide to the customers.
Having said that, a good test can give you good validation on your product/service. If it's a product, you can use crowdfunding or Quirky to test the interest. If it's a service, you can create a signup page, and raise awareness it via well thought out plan that includes blogs, partners, paid advertising and other marketing channels. In addition, there is no substitute to talk to people in person (offline). It will help you describe your idea in detail to a potential customer, and gauge their true reaction w/ verbal feedback as well as facial expressions, body language etc.