In my opinion, Oda has been all over the place with the ACoC stuff, and I honestly believe there are only two ways to confirm ACoC:
1. By direct statement (Kaido to Luffy on the rooftop)
2. Fodder passing out in tandem with an attack (Shanks' Kamusari, Zoro's KoH vs. King, and G5's first appearance, for example)