I have alkways called mine by their first names. basically how I do it is I ask for an appoinment card and if the therapist is not a private practice they have always wrote their names on the appoinment card for example the appointment card says - date and time of next appointment with... and my therapist writes in august 1 2:00 and jane or jane doe. if they write just Jane - thier first name on the card I know that its ok to call them by their first name.