This thread might help. Books on Southwest Indian Jewelry?
The only book I have with hallmarks is a Hopi Silversmithing book. There are some other sites online with hallmarks, but they don’t have quite as many. Here’s one…https://www.medicinemangallery.com/native-american-art/indian-jewelry/identify-native-american-indian-jewelry-hallmarks
Once I think I find a match, then I look online at reputable sites/stores (I definitely don’t rely on Etsy and eBay) to see if my piece looks like other items by that artist. And of course I ask on here!
You might also check bookstores in the Southwest, maybe even some of the National Park bookstores. I’ve gotten some books at parks. Maybe check the site for Mesa Verde National Park, Chaco National Historic Park, or Hubbell Trading Post National Historic Park (I got a couple good books there). Or museum websites (the Heard in Phoenix, the Eiteljorg Western Art museum in Indy, etc).