Ask HN: Customer validation before building a product

One place might be Q/A or support forums where the discussion can be a reasonable proxy to your project. For example if a recurring complaint or feature request is 'how can I xxy' or 'can you do xxy'. If your proposed product can do xxy, or reduce the hassle or cost of doing of xxy, that might help validate the need.

Polite, personable cold calling, visiting industry conferences/exhibitions, paying for advertising or at least look at the numbers of searches for your key words.

Follow industry experts on twitter, linkedIn.

If applicable, small batch direct mail, advertising in popular niche publications

I've always found if you make it clear you're not selling anything (yet!) and are genuinely interested in solving their issues you can get people to provide feedback quite easily.

If you're sending 100 emails and only getting 1 reply, something is horribly wrong with either your targeting or your messaging.