I am asked this question many times. While it is a rather large question given the amount of collectable military items and arms, it is valid. Things can certainly be found on the internet, but some things really need a book to get the information. Nobody knows everything (except my wife), but I’ll try to give some ideas and reference material that may help.
For American antique arms, the first place I look to identify a gun I don’t know or a specific model is Flayderman’s Guide To Antique American Firearms by the late Norm Flayderman. The last edition is the 9th printed in 2007 and while the pricing may not be current it is filled with information to figure out what you have. I use this one all the time as a starting point as it is easy to search if you find a manufacturers name on an antique arm. But there are more than just American antique firearms.
Rhode Island is home to one of the most prolific publishers of research material for arms, swords, and militaria. Mowbray Publishing is located in Woonsocket and is run by second generation historic arms publisher, Stuart Mowbray. Between he and his late father Andrew, they have published an amazing amount of reference material on all sorts of things. As I catalog, I find the majority of books I pull off the shelf for more information are published by Mowbray. Swords from the 17th century through the 19th century are covered in numerous volumes, muskets from the French and Indian Wars through the Civil War period, European firearms, weapons of World War I and II, as well as shotguns, Colt’s, Volcanic Arms, and many, many others. All of Mowbray’s books have hi resolution photographs that make identification easy and cover every detail imaginable.
If you have more than just a passing interest in learning more, Mowbray also publishes a fantastic magazine six times a year called Man at Arms Magazine for the Gun And Sword Collector with interesting articles on all sorts of arms as well as what shows are coming up and other related events. Many of the articles are written by world-famous authors as well as museum professionals from all over the world and I look forward to reading every issue. Mowbray’s website is www.gunandswordcollector.com
As I mentioned before, I do also use the internet to help identify or clarify some of the items I am researching. Sometimes a quick internet search with a keyword can help get the ball rolling and is a good way to start. Just be careful of some of the pricing you see. Anyone can ask anything for an item and sometimes the numbers are not realistic and in line with the current market.