There's no substitute for research, especially when you want to avoid buying fakes. I have found the following references valuable. None of them contains all the answers, and some of the authors and acknowledged experts disagree with each other sometimes. Please bear with this rather long list. It's not meant to be comprehensive, but it does cover a number of subjects, especially medals and badges.
Web sites are some of the best sources of current research and identification of fakes.
I've included links to some that I think are useful. A lot of information on medals and badges. Lots of info on daggers and dagger reference books, with a discussion forum. And Their Values, by Adrian Forman. R. James Bender Publishing, 2nd Edition 1993.