Another way to do this is to join some health forums, (perhaps you are already part of some?) and give good content and answer questions. After a while, ask them what they would like to see answered in an information product. Same as above, what is the number one thing that they would want to know about.

If you offer your product, as said before, for free for those who give suggestions, you should have great answers.

I've done this for mine (In the IM niche) and I got a decent result.

Rob