Is there any way to tell if an assassin is in trance?

Sort of? They will have (potentially) more dam redux than normal. The trend at hero right now seems to be *only* fight in trance, so assume they're tranced.

Duration/Cool down?

I was to say 8 hours every 24, but it could be less on both of those.

Can they call it in battle?

Not that I'm aware of. I *think* they can't have PK adrenaline even, but they're a stealth class, so they get to pick their fights.

Is there an echo?

Not that the room can see, AFAIK, but also they won't be in the room when they call it.