Basically:
1. The year is stamped on top of the chamber on military lugers. All ORIGINAL Lugers were either 9mm, or 7.65; 30 cal Luger. If it is a .38, it's been re-barreled.
2. Most 1936 Lugers were made by Mauser. Krieghoff also made them, but they are rare.
3. Check on this forum or
http://www.wonderhowto.com/search/luger/ for disassembly
4. Mag serial numbers are stamped on the bottom. Some bottoms are blank.
5. Read the New Collectors pages here.