The mounts on an FL120 are shot when the rubber collapses to the point of metal to metal contact. This is easily discernible by eye. I don't know about the mounts used on other engine types.
One of the "experts" who explained to me how to judge the mounts on an FL120 and also told me the average life expectancy of these mounts is Bob Smith of American Diesel, formerly of Lehman. My guess is that he knows just a wee bit more about these engines than pretty much anyone else.
Although he does still (I assume) recommend adding Marvel Mystery Oil to diesel fuel to increase the lubricity when studies of fuel additives by the truckng industry show that MMO significantly decreases the lubricity of diesel fuel. So maybe he's not quite the expert he's believed to be. Still, I think I'd be inclined to take his advice about FL120 engine mounts over anyone else's. He's certainly proved to be right on the money with the other suggestions he's given me on how to improve our engines.
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!! Some of us study how the system operates and make an informed failure analysis based on facts. Oil sampling, compression tests, voltage checks and other available tests allow some of us to pinpoint a cause of failure and occasionally improve on an original system to extend the expected service life in the future. Honestly though- a large percentage of failures are the result of an incompetent prior repair. Many of the remaining break downs are due to neglected maintenance or damaged caused by an operator. And in the end- almost everything on your boat, car or airplane is ultimately still a wear item. From the door latch to the water pump. Mostly the good techs are good guys. Being honest from the start MAKES you improve. You have to live with your mistakes 
