Hi, Katie,
I doubt there's anything useful you can do with a multimeter to test the horn (at least without dismantling it).
I agree with Rodd: Connect the ground side of the horn to the negative terminal of your battery (any 12 volt car battery), then use another wire to briefly connect the positive terminal of the battery to the other side of the horn connections. It should beep (or honk...).
If it doesn't, then it might be time to look inside, where the multimeter might be useful.
-David