The best way to recognize a fake Green Card is to do what the Federal government does with people who detect counterfeits: become so familiar with the real thing that a fake will 'pop out' easily in contrast. Many things have been done to try to stem the growing problem of fake Green Cards. For an illegal alien, seeking to find employment in the U.S., a fake Green Card may mean the difference between working and not working. Employers are legally prohibited from hiring foreign nationals without Green Cards or valid Visas. This means a person must either enter legally and get a Green Card through the right channels or buy a fake one on the streets.
Since fake Green Cards are big business in a country with millions of illegals already here and more coming, there are people out there willing to go through a lot to make sure the cards look like the real thing. So far, they have been very good at it, and finding the fake ones is difficult. Add to that the idea that employers are not trained or required to look for them beyond an ordinary perusal, and you're in business. However, becoming totally familiar with a real Green Card should help a person be more likely to spot a fake.
Two "card mills" were shut down in Indianapolis not long ago. More Mexicans from deeper in the remote parts are heading north which could mean that word is spreading about the easy availability of fake Green Cards. Added to that the amount of money they can make here as compared to their home, and you have an epidemic.
INS has recently effected a new program for Green Cards that involves a 10 year validity/renewal. They are also working to create a Green Card that is more "physically complex" and encoded than current cards. The problem is that with all that money at stake, someone somewhere will figure out how to counterfeit it.
In the meantime, employers and others need to be vigilant in looking for the counterfeits and prosecuting those who violate the laws of the United States.