Yes physical therapists can and do diagnose people who they see.
The physical therapist treats and diagnoses people of all ages including ages, from newborns to people at the end of life.
Physical therapists can treat and diagnose patients who have injuries, disabilities, or other health conditions that need treatment.
Not only that but physical therapists can and do also treat patients who just want to become healthier and to prevent future problems with their health.
So if you do have a condition with your back or other part of your body that makes it difficult for you to do anything or you have pain then a physical therapist can diagnose those problems.