Ultimately ideas aren't worth a lot, it's honing those ideas into something useful that people are willing to pay for that makes the difference. Plus people tend to be pretty arrogant and have a bunch of (usually terrible) ideas of their own so why would they want to bother with yours? :)

In terms of validating in "stealth" though, if it's a B2C model there are plenty of potential customers out there you can happily share your solution with and have close to zero risk of it spreading any further. You could probably beg them all to tell ten friends each and your secret would still be safe. Trust me, you'll be wondering what you were worried about if you end up proceeding and then have to actually incentivise the distribution of your idea/product!