Ask HN: How can one gauge interest for an idea?

Read up about making good landing pages.

Make a landing page selling your idea.

Have a 'Sign up' button and a 'Free trial' button.

Make it go to a page where you mention that you're not yet ready for release and ask them for their email address and permission to let them know when it's ready.

Run a Google AdWords campaign using various keywords that point to your landing page.

If you have different variations of ideas, have different sets of keywords go to the landing page with different parameters and then use different text to describe your idea to those specific markets.

Use the list of emails you collect to determine how good the market is.

Don't spam those people unless you really do have your product ready.

Screw making a landing page for an idea that isn't fleshed out into a product. Try talking to people you think might use your product, and to people who should want your product (not necessarily the same groups). Unless your target market consists of you, your friends and people like you and your friends, don't assume you know anything about your target until you actually talk to them. Do cross-category evaluation to understand the space you're working in - assume you won't get more than 5% of the total, ever.

I second all the suggestions of @statictype. Find various ways to receive unbiased opinions from ppl who fit your target user group. Don't invest too much time UNTIL you've received enough validation.

As a compliment to that, I highly recommend that you read The Lean Startup by Eric Ries. Best of luck.