Has your camera a loose mirror, though? (if it is, you should see it by just taking the lens away and taking some shots without film, just to see what the mirror does).
I would make some test shots with film focusing at infinite with a normal lens. If the actual picture is in focus and the viewfinder image is not, that means something in the viewfinder is out of specifications (mirror, focusing glass). If the actual picture is not in focus (and it should be regardless of how the image appears in the viewfinder, as you focused at infinite) that means the problem is somewhere else (film pressure plate?) because the mirror, or the viewfinder in general, has no effect on the focus of the final image.
For the test to be reliable, tripod and flexible cable (or self-timer) are mandatory. If your camera doesn't have mirror lock-up, you should use shutter times shorter than 1/60 (or longer than 1 second).
If this test shows a picture not well in focus, then you probably have a problem with the film pressure plate. If the test shows a picture in focus, and the image in the viewfinder not in focus, then you have a problem with the viewfinder (not necessarily the mirror).