Research the closest adjacent categories. It will help you highlight where the gap in the market is, and why your product/service can fill it. Also do research on that specific gap, in terms of who the primary buyers are and what the possible market value could be of the category you are developing. ie - Who will buy it? Why? How much would they be willing to spend? How often?
If you have a specific product or service developed, put together a focus group of people who fit your buyer profile and ask for feedback. There are companies that can put these together for you if you don't feel comfortable doing it yourself. Focus groups often ask for something in exchange (typically a combination of flat fees and/or paid meals and/or free products). While anecdotal, it would be very specific to your business.