The ping test got me worried. A friend, questioning how genuine a couple of my .9999 maples are, had us both pinging in the wee small hours. Sterling silver seemed to ping longer, which I suppose makes sense if it is harder, like the note of a harder bodied electric guitar lasting longer than a softer wood electric guitar.
Also the presence of milk spots made us wonder, but a this video by Drutter says that the milk spots are a result of the annealing process. A faker would probably try and create a perfect shiny coin without any defects.
Probably a good idea to search ebay for a bottle of acid tester.