Interesting question.  When I was in school we actually were required
to take a few patients to a clinical lab and run some clinical tests.
One of my patients had a blood glucose over 300 mg/dL, and was spilling
sugar in her urine.  She had no prior known history of diabetes.
    Generally if a patient gives no history of a family doctor and they
have dental issues I will certainly ask about it.  It's tough to make it
a pre-condition of treatment.  If you're treating an adult you have to
assume it's in their best interests to be honest in their health
history.  But people sometimes are dishonest, and you don't want to find
out that a patient has uncontrolled hypertension or is on coumadin after
an extraction.
    Also what should be obvious is that if you're treating a patient with
periodontal disease, your clinical outcome will be better if the patient
with diabetes has it under control.

Steve