Besides being noisy, how can you tell when strut mounts are bad? I've changed them before when the've gotten noisy, but I know that they can be bad way before they get noisy.
I also have a couple used strut assemblies in the garage, how can I make sure the mounts are good before I put them in?
Noise,and freeplay are the easiest way to diagnose.
For what new ones cost,I always just replace them,they are cheap.I would never bother installing a used one.