How do you know if the gun you have is clean and has not been stolen?

There is no easy way for a private citizen to check the legitamacy of a given firearm. The stolen gun database (NCIC) is a Federal computer system open only to law enforcement. MOST states do NOT have a system nor a requirement to register guns. Local law enforcement MAY be willing to run your gun through the NCIC system, but will want to have possession of the gun in case it IS stolen.