I don't know if there is an exact measurement to determine the goodness of the mounts. My mechanic and I have always just taken a look at them and if they seem to be collapsed any or you see cracks or pieces of the rubber mount gone, it is a good indication that you should replace pretty soon. Also, you should notice quite a bit more rocking of the motor if the mount is giving away or needs replacement. You can also take the steps above and combine it with the amount of mileage on the car (mounts) and pretty much guess whether they need replacing. Sorry, could give you more specific information.