If its the active one that goes, the engine will shake a bit more, you can look underneath for tell tale red oily stains- sign that the oil the mount is filled with has leaked away. The other one is solid and unless you lift the engine to check for the bolt if became loose from the rubber mount then there is not much to see. They mounts are also known to break the alloy part (the bolt breaks away and gives a similar noise like a bad balljoint) and will be felt at take-off or sudden stops as the engine lifts and drops with a clunk. I would fit audi rs4 solid mounts, they are reversible, just ignore the fault code when you scan the car!