Real Leicas have:
-quiet shutters (the fakes sound like a tank coming down the road)
-base plates that have little play when on the body (the fakes usually have a lot of slop)
-quality fit and finish (the fakes won't feel smooth on any of the controls, the chrome is either
rough or extremely bright)
-matte leather/vulcanite (the fakes are usually have a bit of gloss to them)
-little odor (the fakes smell like a mix of machine oil and sour milk)
-smooth screw mounts (the fakes usually hesitate or bind when the lens is being put on)
-if it's a real Luftwaffe, Heer, or Kriegsmarine model, it will usually show signs of use (often heavy use) and the chances of actually seeing one of these outside a major city (Tokyo, for instance) is very remote. Fake military Leicas always look new.
Also...do not buy a Leica from Ebay...buy it from a dealer that you know and trust. A good camera store will have at least a 10-day return policy, and will know something about the camera you are interested in (if they can't do either, don't buy from them).
Hope this helps