Sometimes you look at the date code list and you look at the proofmark, and you find the one you think that's closest to it--but they're often not a perfect match in appearance. Some are really clear, and it's easy to say that's an a, for example. The underlining really helps to tell the 20's-40's guns from the 60's and later guns. But those strange Greek/Cyrillic letters they started using in the 50's can be really tricky.