Can hypnosis help us know the main cause of psychosomatic pain?

Possibly...: Hypnosis is very theoretical in nature and to the extent it is effective, the psychological mechanisms by which it might work is not very well understood. Better understanding of the actual mechanisms of hypnosis would likely provide insight into psychosomatic disorders.

Sort of: Patients in trance, when asked carefully constructed questions about the cause of their psychosomatic pain will give an answer. That answer may or may reflect reality as we know it or may be a metaphoric subconscious construct. In either case, a skilled therapist can use that information to help the patient achieve relief of the pain that used to be there.