Hi, are we talking about a broad market product or a niche product? Also, is there already someting similar on the market?
I would say a broad market. Similar in some ways as people are using various current tools for the need.
The number of customer (or often whole familly as many things can be shared) and the price is a good start. I would add that the price alone is not a perfect metric, sure there is a max price most can spend (as good is something, the market is small for 100,000$ tools), so you need a cost/benefit(value) annalysis not forgetting the resistance to change factor (a market can be hard to gain just because of new learning curve and general inertia in decision process).
You can get data on diverse things (i usually use Statistic Canada, there is plenty of agency like that), you can do a "focus group" or surveys, again many company specialize in doing that for you if you don't have time.