How to ascertain speaker damage?

The simple answer is to plug them in, listen and determine if they are ruined. It will be fairly obvious if they have been damaged once they are turned on. Also, look at the drivers to see if they have been hurt in any way.

My only suggestion is to move your hi fi system to a room your housemates don't really have access to, like a bedroom. I went through a similar situation, only my roommate never damaged anything because my stuff was always in my room instead of a main living room

it's probably OK
Do some critical listening for awhile, then throw your balance all to one side & listen mono, then to the other side. Or switch around the two speakers & listen from the same side. Any issues will likely manifest themselves, or not if all is OK.
Then give roommate the walking papers.

Try to drive it real loud one after separately and see how they accept the juice.
If notice that it drives an amp to clipping after 10pm volume level than it's probably the voice coil.

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. :]