I'll write it here just so I wouldn't be the only who knows.
You can actually detect when a unit is summoned by a spell.
When an effect of ability event starts the ability hasn't yet done anything, but it is inevitable. At that time put a 0-second timer.
This timer will end when the ability has already done its instant effect.
Between those 2 times, any unit that appears on the map is definitely a result of the spell being cast.
You can detect most summoning spells with this method, but the additional spawns of lava spawn don't work, and neither do doom or inferno.
Raise dead does work and so does water elemental.
I am currently working on the final edges of the items, and then I continue with quests... after that, an early alpha should be pretty easy.