You really don't care whether someone else out here has a product like what you're thinking about (at least not from a legal perspective) - after all, we live in a competitive world where Burger King and McDonalds can both sell hamburgers without infringement concerns. What you DO care about is whether there are design ideas embodied in your invention that are already patented by someone else. So what you need to do is a patent search. The US Patent & Trademark office has an online facility for that - it's free but not very user-friendly and takes lots of perseverence. Choose key words that are as specific as possible to the key technology in your invention. If you find a patent that is close to your invention you can research other patents that referenced to broaden into related inventions. It's not a fool-prooof method, but in a few hours you'll have a pretty good idea about the state of the art. To evualate whether your product may infringe you have to carfully read the claims at the back of the patent document and see if the physical embodiment of your product infringes on what's disclosed there.

You can start here: Search for Patents

Alternatively, you can use software that is intended to make patent searches easier. See for example: PatentHunter 3.5 - Patent Downloading, Searching and Management Software