So i assume your refering to damage from stuff like shockwave.
In theory there should be some sort of generic way to detect the damage dealing unit, because units definitely know what unit killed them regardless of the means. But I guess that only blizzard could make that piece of data accessable.
If your requirements are quite specific, there may be a somewhat easy method. (for example if only direct damage spells are stuff like stormbolt, you could easily use arrays and stuff). But if your talking the full range of damage spells I think it'd be pretty much impossible, especially with them darn AoE skills. | 
