Leica fakes are quite sophisticated and most of the visual appearance can be replicated quite convincingly, but beneath the skin it's not so easy, without expending more effort than the faker is prepared to. So here are three things to look for. These differences are not definitive, but these are hard to fake and easily spotted.

The first telltale point is the engraving on top. Leica engraving is very fine, with delightful sweeping smooth curves, and no appreciable swirling from the engraving process.

At left is a genuine 1938 Leica II.

Below are two close ups at exactly the same enlargement factor, on the left is the same 1938 Leica II, and on the right is a FED 1 fake. Now this particular FED isn't pretending to be a Leica but a military version of FED, however, the engraving is poor as it was made by hand and is indicative of the sort of thing you should look for.

If you click on either image it will download the high resolution original for very close comparison.

Next on the list of easily spotted visual clues is the rangefinder follower. Unscrew the lens assembly and look at the top of the opening of the body. On the Leica the follower is a wheel free to rotate that follows the back edge of the lens barrel as it screws in and out during focussing, in turn it causes the mirror in the rangefinder to move and brings the two rangefinder images in and out of convergence. This wheel is beautifully made, perfectly concentric and spins smoothly as one would expect. FEDs and Zorkis did away with this, and have a simple static follower point, it works, but is not as smooth. Below, left to right are A Leica II (paint's a bit tatty), FED 1 and a Zorki 4, which whilst it had deviated away from being a Leica copy, shared very similar internals to earlier Zorkis.

The film transport and shutter mechanism gears on a Leica are machine cut, and even an old fairly neglected Leica will have have a silky smooth advance, unless there is something seriously amiss with it. The same cannot be said of either FED or Zorki derived fakes. Their gears are stamped out, whilst this is a perfectly acceptable mass production technique it can lead to a gritty, stiffer gear train unless additional finishing is undertaken. All the FEDs and Zorkis I have held exhibit this tell tale grittiness. This is very apparent if you have an example of each to hand, however it's not possible to see easily.

The shutter release action on Both Zorkis and FEDs is noticeably stiffer than a Leica. The reason here is that that Leica finishing is much more refined. This standard of finishing can be seen on the sprocket drive shaft quite readily. Another visible, with some difficulty, Leica difference are two stop pins fitted to the Sprocket drive shaft. Both FED and Zorki cut corners and used a single stop pin. This is visible with some effort and the pins can be seen protruding under the film sprocket shaft. To see, remove the bottom or back, depress the shutter release, you will see a gap open as the release moves the escapement. With the shutter depressed, wind on (the shutter won't cock, curtains advance or wind on stop) and count the number of pins, there should be two on the bottom of the sprocket shaft, 180° apart.

Below is the Leica Sprocket on the left, one of the two pins is visible. Note the pattern made by the machining process, the standard of finish (the milling marks go in the direction of rotation) and the angle of the sprocket teeth ramps, now compare this with a FED 1 on the right. Again there is a pin visible, but there is only one, you will have to wait one full rotation for it to come round again. Note the poor finish of the sprocket wheel, it is roughly ground along the axis of the shaft, the teeth are narrower at their base with steeper ramps and there isn't a groove either side of each tooth.

Finally, below are images of two genuine Leicas, on the left a Leica II from 1938 and on the right a Leica III from 1939, clicking on either image will take you to the full rotational image allowing you to view the camera from all directions. Both these cameras had hard lives and the original covering has been replaced.