Q: I have a client with a unilateral lisp. I have noted symmetrical facial posture at rest and non-speech tasks. However he exhibits a unilateral retraction of his cheek that corresponds to his lateralization. I realize that lateralized productions reflect the tongue position more than the cheek position, but I am trying to determine what is happening inside the mouth during these times. How do I tell what he is doing with his tongue?
As you have rightly surmised, cheek lateralization is a secondary effect of inappropriate tongue position. The tongue position is driving his sound laterally and he is pulling the cheek out of the way in order to allow the air to escape to that side.
To assess the tongue position and the direction of the groove, use a straw. Place one end of the straw against the teeth at the central incisors as the client makes a prolonged /s/. Then move the end of the straw slowly from the central incisors to the molars on the right, and then repeat on the left.
You will be able to hear where the airstream is rushing out, and that will tell you in which direction the client is positioning the channel.
I describe this procedure in my book called Frontal Lisp, Lateral Lisp.