You can inquire with the front desk if the facility allows or encourages gratuities. It would depend on several factors, such as: Is the therapist paid on commission, or paid hourly, or paid by insurance?
Do they rent the space individually, or are they an employee?
How often do you see them, weekly, monthly, etc?
Either way, tipping is generally appreciated, and never required. If tipping is not allowed, (or is factored into the cost of the session) a nice card is always appreciated!