When kids are not focused on the words but on the message, which is our main goal (because then we know that real acquisition is taking place), we can tell that this is happening by simply looking at their faces. It is like being in a movie theatre and being able to look back at the audience from the screen.
If the students have expressions on their faces that indicate that they are getting a kick out of each new event as it unfolds, then we know that we are pretty close to the goal of creating real, even compelling, comprehensible input. If disbelief has been suspended, and time even seems slowed down to allow a kind of absorbing of the message by the kids, then we are doing real CI.