Some ways are easier to tell than others.
If any of them are broken you need all new ones and the engine
realinged. You don't replace just one or two. If they break you waited too long. This can also throw the alignment off. You can do this in the water
but if you need to replace them it's good to check the shaft and cutlass bearing at the same time. Poor mounts can contribute to poor alignment and to scoring on the shaft extra stress on the cutlass bearing. If it's a 1989 boat I would expect to have replaced the mounts at least few times if not more. They do wear out.
Given the boat is a 1989 it would be a good idea to go over the whole setup if you replace mounts since it requires realignmnet of the shaft. Alignment is a job you want as perfect as possible. Getting it to .0015 inches is possible even though specs may be as much as .003. Getting it that close will be something you can notice and hear. Not every mechanic
can do it that close.