[mjs00]

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.

[uast23]

Yes, thats pretty valid. But I have seen people trying to promote their product on support forums facing a huge backlash. Although I can assume that if my product really has something valuable to offer, trying it won't be worthless.

[mjs00]

My suggestion more around understanding generally if there is potential demand which is what I thought your question was. I wouldn't promote your product there directly.