Only plausible solution requires orb effects on every unit in your map. Just add some orb effect (Orb of Corruption is probably the easiest, since it's invisible by default, and its effect can be nullified) to all units, and when they are damaged, check if the Orb of Corruption buff is present. If it's not, you should be able to assume that the damage dealt was from a spell.
If you need, I can make you a demo map showing the effect.
Caster dummies should ALWAYS have locust, otherwise players can select them Oo and yeh, remove the projectile from the dummy spell, or place the dummy in the exact position of the casting unit.