As I mentioned in previous posts, it's generally the last person holding counterfeit money who ends up losing. There have even been reported cases when credit unions have given members counterfeit bills. So it's important to closely check any bills that you receive from any source, especially bills that are over $20.
This MintLife Blog infographic
covers the history of what the government has done to inhibit counterfeiting. It also includes tips on how to check for the security features that are contained in today's bills.