The battle message being:
"Pokemon is so in sync with Trainer that it landed a critical hit!"
Does a Pokemon land critical hits just randomly anymore, or are crits landed always because of Pokemon Amie?
Ok so I'm going to explain...
There are still Critical hits even if the Pokémon is very well looked after...
Pokémon with high affection will land Critical hits more easily and it will state that the Pokémon is so in sync with its trainer it landed a Critical hit! A small heart rises from the Pokémon's head.
So if a Pokémon with high affection lands a Critical hit it will always state it's on sync.
However, if the Pokémon has lower affection it can still have Critical hits but it will state 'Critical Hit!'
Hope I Helped! =]