Last time I was at Saturn they said one of the motor mounts was just within
spec and would need attention eventually. I left it for another day. But I
was wondering, what do you see, feel, or hear when motor mount replacement
is necessary?

I have noticed recently while idling in neutral that the motor was running
rough. When I paid close attention I realized the motor was running fine,
but engine vibration seemed increased. Is this what happens, more shaking is
transferred into the frame when the mounts fail? Seems to make sense to me,
but what do I know!