How do you look up the serial number on guns?

Your question is somewhat vague. WHAT do you want to look up? To see if a gun is stolen? You cannot do that- the database (NCIC) is restricted to law enforcement. However, check with your local police to see if they will check. Want to see when a gun was made? No one answer. For some makes, there are public databases and reference books. Some makers will have that available on their website (Ruger, Browning). For some, there is no information.

If you will post make, model and serial number here, or in a private message to me, will try to help if you are seeking date of manufacture.