Ask HN: How do you test for a market for your idea?

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.

Create a landing page that garners interest and asks for the users email and lets them know when the product is finished.

http://www.mailboxapp.com was a recent example that started testing the market with no product.