Plug your social security number or credit card number into this specialized search engine and it will look through the universe of stolen numbers to tell whether your number has been compromised. I tried it and it turned up bubkis. For $8/year, they'll continue to monitor a list of your numbers and email you if something fishy ever happens.
So how do I know this website doesn't just harvest everything you type and then use it against you? First, they promise they don't. Second, they are backed by Draper Fisher Jurvetson, a leading Silicon Valley venture capital firm.