Start with the information stamped on the gun. Usually there will be a manufacturer's name or a trade name and a model name or number. Guns manufactured in Europe will usually have proof marks. With this information you can search various books available at your library: Flayderman's Guide to Antique American Firearms, The Standard Catalog of Firearms, Modern Guns, etc. Or you can post a description that includes all the information you found on the gun on a website like this and someone will probably recognize it.
Gun shop, gun show, old catalogs
Old gun magazines, the library, the internet, old books.
Library, old gun publications, internet.
Old gun catalogs, Blue book of gun values,
Old gun catalogs
Blue Book of Gun Values, old gun catlaogs, gun shops, gun shows.
Old gun catalogs, on line auctions, gun shops, gun shows, libraries
L Blue Book of Gun Values, old gun magazines/catalogs
Blue Book of Gun Values
Gun shop, old gun catalogs, blue book of gun values.
Blue Book of Gun Values
Old gun catalogs, library, gun shows, gun shops, Blue Book of Gun Values, Standard Catalog of Firearms.