If the camera has been idle for a long time -open the back and see how the curtains travel. At the end of the curtains there is metal bar crimped on to it (running vertically). Occasionally you have a tired 2nd curtain and the metalbar does not go all the way in behind the filmgate. When the camera is released and you look through the back, all you should see is curtain material, not the metal bar poking out on the left of the filmgate.
Also hold the camera up to your eye, again with the back open and look through the filmgate. Run through the full gamut of filmspeeds while aiming the camera (without the lens) at a strong lightsource. The slow speeds should be even and the curtain should travel smoothly across the filmgate. The speeds fom 1/125 on should show a progressively "shorter" exposure, but at least up to 1/500 you should see the whole filmgate open (1/1000 too but that one is difficult to judge by eyeballing it). If there is a problem with speeds it is usually too slow at 1 sec. and 1/2 sec and "capping" at 1/250 and up. This means that the second curtain follows too close to the first one and there is no visible light through.
A simple test is to pick a wall or evenly colored surface, run a film through at all the speeds (moderate the light with apertures). You might be going from 1 sec/f16 to f/2 at 1/1000 - but the negative should show very similar exposure across the board. As the M-shutter is a mechanical shutter, you will find that the 1/500 or 1/1000 will most likely have a bit "brighter" image tone as it is rare to have exactly 1/500 or 1/1000 speeds dead on.
While you are doing this test, also check focussing accuracy by simply measure off 1meter (or 3 feet if that is how you lens is marked), 2meters (6 ft) and 5 meters (15 feet). Measure the distance from the back edge of the camera and preferably on a tripod (the exact distance is about 3mm in from the back edge - but unless you do repro work, it is not that critcial. Set the lens at indicated distance and shoot. Use something that is easy to see on the neagtive, a pole, back of a chair or something with empty space ahead and behind it so it is easy to see. Again do this with the lens at f2 or 2.8.
While it sounds more complicated than it is, once you have "wasted" a film this way - you will know how the camera behaves at various distances.
I do this procedure every year with each of my cameras. It makes it easy to see if a specific body is "drifting" towards a CLA.
M cameras hate being idle so shoot away!