Mostly the presence or absence of this "Balance" can only be evaluated across time (through a whole session, or across different sessions) - but the therapist will be looking for "what is not being aired" as much as the content of what is being aired.
It is the therapist’s task to help the family make sense of what feelings are experienced by each family member, as well as highlighting the ways in which miscommunication or misunderstanding (or lack of understanding) of these feelings leads to interactions that maintain family problems. In practice, this requires the therapist to strike a very careful balance between allowing the family to interact ‘naturally’, or indeed actively eliciting habitual and ‘natural’ family interactions around problematic issues, as well as being directive and intervening at critical moments too.
In individual work it is much the same. Here, Anthony Bateman is holding the balance between cognitions (which could become rather dry, and divorced from the real pain involved - a kind of Pretend mode) and the emotions (which could become overwhelming and lead to a loss of mentalizing) in a discussion about a court case relating to custody of a child: