To test if your camera synchs properly with a flash is no problem as long as you can open the camera back. Looking at the frame opening of the camera while firing the flash at a bright surface will quickly reveal if the entire frame is exposed correctly. But what to do with cameras that don’t have an opening back like all of the Leica screw mount cameras?
There is a very simple, non-invasive way to do so. All one needs is a piece of so-called glow-in-the-dark-tape. This should be noticeably larger than the frame opening of the camera. Inserting it into the camera in place of the film is all that’s necessary. With the lens removed, if possible, connect the flash to the camera, set the required synch speed and set off the flash, preferably firing right into the camera and then remove the tape. In a not too bright area, the tape will glow with the outline of the exposed area. This will quickly reveal if the entire frame is illuminated or if the shutter is covering part of the negative area while the flash is firing.
This test will work with all types of flash equipment. Especially with cameras like the Leica IIIf, which have a built-in flash synch adjustment, this test can quickly determine the proper synch settings without the need to hunt down an instruction book.