Answer:
The FBI goes to great lengths to protect what it's identification and real badge look like. The problem is that you really would not know if a guy was a fed or not because darn near no one would know if the ID is legit if some terrorist or spy was impersonating an FBI agent. Heck, I suppose you could easily be arrested by a criminal. It would be easy for the FBI to say the person had fake ID, but how the hell does a tax paying citizen know? I guess you are supposed to just yield to anyone with ID and hope you don't end up dead. Is this how all that homeland security rocket science is supposed to work? Geeeeezuuuus!