Take the back off the camera, plug in the pc cord and turn on the flash. Set the lens to f2.8, point it at a wall and look through the back of the camera as you fire the shutter. If you see the flash when the lens opens, the synch is good. If you don't, it's set to M... or the x synch is off or not working.
I learned this the hard way with a Rollei TLR, after shooting a couple's 50th wedding anniversary. Ever since then, I have performed this test before shooting a job with any camera with flash.