Over time, movement, heat, the ozone, light and chemical reactions will degrade the quality of the rubber and form cracks in the mounts. They may even break all the way through. They will need to be changed at that point.
They way to know if they are broken can most often be felt. When the engine is running and you shift into reverse or drive and you hear a kerplunk noise and feel a harsh movement they are most likely broken. You may also feel a vibration when the engine is idling and the vehicle is in gear.
If there is still any doubt and they like this...change them please! Beep, Beep!