Do some detective work!
Check out who’s already selling what you are thinking! Ask yourself these questions: Does your product create a completely new market? Is your product a variation on an existing product? Who are your competitors? What’s your market space? Does your product have an existing niche to market to?
Spend some time googling your product idea to see if someone else is already doing the same thing. Entrepreneur magazine has some in depth pointers on conducting market research.