When you cast a creature for its bestow cost:
- When you begin casting, it is a Creature spell
- We know this because Steel Golem prevents Bestow creatures from being cast
- When you pay the alternative cost, it becomes an Aura spell
- When it gets placed onto the stack, it is an Aura spell
Bestow triggers things like Prowess. See Matt Tabak's ruling on this subject. However, it does not trigger things like Karametra, God of Harvests' triggered ability. See the ruling on Karametra herself. The Karametra case is confusing to me, because at one point, you were casting a Creature spell, and at another point, you were casting an Aura spell. It seems like both should trigger. So my real question...
At exactly what points during the casting of a spell do triggered abilities check to see if they triggered?