Experience.

Right after Ember disappeared, chaos ensued.

Rai commanded the Transcendents to create a circle around the estate while the rest of the Beastkin and the trio moved out of the ring. 

As the estate was emptied in a matter of seconds, a barrier was cast using the Transcendents' Spiritual Essence as the medium inside. No one could see anything inside.

"If anyone lets her escape, you won't be assigned a room for a month!" Rai shouted as he drew back from the barrier as well. The barrier wasn't professionally made and just needed one of the Transcendents to stop feeding their Spiritual Essence to it for all of it to crumble down.

Rai couldn't feel any Spiritual traces from the Ozul, but having heard his conversation with Ember, he guessed him to be Transcendent as well.

The rattled woman stood alone facing Ozul inside the barrier.