how can I tell when the lower engine - trans mount needs replacing?

Owner: engine and transmission mounts can fail in two ways : they can break, allowing the Engine and transaxle to move out of normal position, resulting in a thump or bump when accelerating or changing from drive to reverse while stopped, or the mounts can become hard or rigid with age, resulting in a harsh vibration @ stops, idling in gear, either failure also may result in a vibration or bucking @ cruising speeds. Failed mounts can also cause a broken radiator, heater core, and Broken exhaust components when the engine movement is excessive. An a/c compressor can also fail in more than one way, it can leak refrigerant, or it may seize internally, causing belt squeal when a/c Is selected, or the clutch bearing may seize, resulting in belt failure and the loss of other accessories which may be driven by the same belt.