Consumerist, part of the Consumer Reports organization, just published
Fakes: How To Identify Astroturfers and Front Groups
Everyone likes to hate on spammers, but they're basically the houseflies of the Internet. Far more insidious and damaging are astroturfers and front groups—those corporate-funded, agenda-pushing people who don't disclose who they're really working for while they participate in online culture and the media. The Center for Media and Democracy has put together a list of tips to help you identify them from real grassroots movements, while Free Press has created a widget that reveals front groups for five large companies you frequently see on Consumerist.
The biggest tip-off, of course, is that there's not a 100 percent clear disclosure of the group's beginnings and location. The Center for Media and Democracy says you should look for physical addresses, and cross-reference them on their astroturfing wiki, SourceWatch.org. (If the address is in D.C., alarms should be going off.) Real grassroots will usually prominently list their chief personnel (but cross-reference those, too) and source of funding.
Special credit to McClatchy News, which exposes front groups working to defeat healthcare reform.
Finally, a disclaimer: Consumer Reports has a brilliant record for integrity and helping people out, and I joined their board to support that.