Avoid wasting time on needless tests. These parts are built to LAST and often do, as @lopez.oscar points out. Just deal with any issue when it actually happens.
Hey @lopez.oscar, your idea to skip the pre-season test is asking for trouble. I read a post by an old timer who said glow plugs often die slowly, so they might work one day and not the next. Finding a bad one before the frost hits means you can fix it on your own time. Waiting for a failure leaves you shivering in a parking lot trying to get a jump. That five minute test with a cheap tool is way better than a tow truck bill. I stick with testing because cold mornings are miserable enough without car trouble.