Well - a start is looking through the vintage ammo for sale on plces like Gunbroker. (empty boxes on Ebay)
There are also collecter books covering old ammo. Some can be worth a significant amount. Like anything collectable ,the cleaner the cartridge, and whether it has a full box, that is also clean, will provide the premium values.
Also like other collectables the scarcity of a particular round can manage a higher premium.
Most of the fun is in the sorting and research.
Years back I was in the retail firearm business and had accumilated some interesting old ammo. My brother fell in love with the stuff so I have him all I had accumilated, and all else that came in. He ended up with a pretty nice collection for free. (he was generous to me on other things so I'm not unhappy)
There is some nice stuff he he has. If your keeping, learn how to preserve.