From the NSSF:
Officials at the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) are warning that some private gun owners and buyers have fallen victim to “sellers” fraudulently posing as federal firearms licensees (FFLs). Private gun owners and buyers can protect themselves by accessing ATF’s FFL eZ Check. FFL eZ Check allows private parties and other FFLs to verify if a federal firearms license is valid. The user needs only enter the first 3 digits and the last 5 digits of the FFL being verified in the blocks provided and select the SUBMIT button. Selecting the RESET button will clear previously entered data.
FFL eZ Check does not validate Type 03 (Collectors of Curios and Relics) and Type 06 (Manufacturer of Ammunition) licenses. NSSF urges all licensed firearm retailers and private parties concerned about lawfully transferring a firearm to use this vital tool.
FFL ez Check can be accessed here.