Legally, a dentist or ophthalmologist can and should make a referral if
they suspect diabetes (for example).  I'm not certain that diabetic
retinopathy has a specific enough clinical picture to make a diagnosis.
 In any case, it is a condition that is in some cases treated by
retinologists, and certainly is not legally outside the scope of medical
practice.
    A dentist may suspect diabetes based solely on the periodontal
condition, but periodontal disease is a far less specific marker for
diabetes than is diabetic retinopathy.  When we were in school, in the
oral diagnosis section we sent our patients for routine blood tests, and
one of my patients showed a blood sugar of over 300 mg/dL--obviously a
problem.  She was informed and advised that she likely had diabetes and
should go to her physician.  Hopefully she went and got treated.
We don't routinely have bloods drawn for our patients, and I don't
generally send a patient directly to a clinical lab.  I send the
patients to their physician, will sometimes call the physician.  I am
not going to manage a patient's diabetes, and there's no reason for me
to run glucose tolerance tests to diagnose it.
    I actually had thought that diabetic retinopathy was generally
considered irreversible.  I'm glad to hear otherwise.  I also hope that
your general physician checked the other possible complications of
diabetes, such as your renal function.

Steve

Good eye doctor but some symptoms, such as diabetic  retinopathy,
may be pathognomonic whereas periodontal disease is not.