How to gauge interest in an idea?

I've recently come up with a few small ideas for starting a business, however I'm hesitant to pursue them much right now because I'm not sure what the interest/response would be. One would be an online-based store, the other more of a local event.

When starting new projects, what do you use to gauge as a level of interest? I already have a general demographic, but am a little unsure of where to go from here... any help would be appreciated!