Over time I must have stripped the screw on my mirror because it now turns way too much and it's useless. I can't even keep it tight; highway speeds make it swing around into my arm.

Does this mean my mirror screw is bunk, or the female housing? And how would I go about testing which? My bike is currently stored at a friends so I can't look at the mirror screw now, but I assume it would be obvious if that was the culprit.

Thanks

See if it will unscrew completely out of the handlebar mount, and then inspect the threads.  Usually the steel threads will be OK, but if it is a thread problem, it will be the aluminum threads that will strip out.

If they are stripped, you could rill out larger and re-thread with larger thread, and them make a reducer to allow mirror stem to handlebar.

Also, if both threads are still good, it could simply be, as someone else is suggesting that you need to tighten down the locking nut.

If the threads are just slightly damaged you might try a wrap of teflon tape around them to fill the gap.