How to ascertain speaker damage?

I'd buy a hifi test disc. I certainly wouldn't see how they play really loud ... that in itself might damage them.

To test one speaker at a time you must have a balance control ... don't disconnect speakers with the amp turned up or you might fry the amp.

Sean, I had the same situation dropping the small monitor aka Kef C10. The volume arround 10pm isn't realy critical for the right speaker/amp combo and should be taken with ease. If the balance control isn't present you can disconnect the other channel with interconnect and still try to drive it. In my case once I've turned the volume level at 9:30 it started clipping while the other monitor took it with ease. After I've removed the driver there was no hope that this driver will ever sound -- the voice coil was fryed.
Yes the mechanical stress can damage the speaker internally so in the future if you're sending someone speakers send it in original factory double-box and order it if you do not keep it.

For punishment I would make him sit and listen to a Bose Lifestyle system until he begs forgiveness. :]