Check this site : http://www.ideaswatch.com/
you will post an idea there and see how many watches (likes) it will get
This can give you a feedback on how good/bad is your idea
Also, away from the internet, you have to talk to someone of your potential customers, see what are the problems he usually faces, what is he currently doing to solve those problems.
And good luck, it is nice question btw, I am waiting for other answers..