If you want to go the most official route possible, head to the FBI's site, http://bit.ly/FBI-checker. With the FBI, you know you're dealing with the most trusted source on this matter — it's the agency that caught the people who began this madness — but its method of checking is also a bit time-consuming.
If you want something simpler, you can also check your computer at http://www.dns-ok.us. The FBI links to this site, so you know it's trustworthy too. If your computer is clean, you'll get a big green image. If it's not, you'll get a red screen. The only issue with that site is that sometimes it doesn't work. I've tried it three times on two computers, but it worked only once.
If you need to try another site, McAfee has set up a site too: http://bit.ly/Mcafee-checker. It seems to work, and it's simple. If you're all clear, McAfee will even congratulate you.