Linehan's model is evidence-based, so those who are certified must replicate the model of DBT she laid out (which includes group as well as individual therapy plus coaching calls) in order to be able to say that it meets the criteria for DBT.
Dialectical Behavior Therapy for Adolescents (DBT-A) is a manualized, 16-week behavioral treatment, that includes concurrent individual therapy once a week, family therapy as needed and a multifamily skills training group in an outpatient setting
The DBT treatment in my area has a teen program that includes parents in the skill-building part of the group sessions. There is also a list of DBT therapists who are certified so people can check to make sure that they've been trained in implementing the evidence-based model of DBT.
The author of Buddha and the Borderline talks about how DBT worked (and didn't work) during her own recovery from BPD symptoms. First she was in a watered down DBT group, then group therapy with CBT, and then a bonafide DBT model, where she really locked in. It's helpful to read about it from her experience. The individual therapy and coaching calls had a big impact from what I remember.
I would ask the therapist if he or she is DBT certified, first. And has he or she trained with Linehan.