Don't over complicate matters. If your doctor did indeed diagnose you with DM II, and stated that diagnosis in your medical records, for service connection purposes, you are diabetic. Makes no difference how he/she came to that conclusion or what test was or was not preformed.
As far as getting it service connected, that is a different matter and will involve more medical opinions in the form of multiple Nexus letters and an event in service that can be as likely as not identified as the cause of your diabetes.
A1C is really the standard and will show right away. It's a essentially a glucose indication of the past 2-3 months. You'd need an opinion from a doctor of potential undiagnosed diabetes from 2002. You might persue that. They accepted my exit exam with Metabolic syndrome as proof though I was diagnosed with in a year of discharge and my doctor's records had the "date of diagnosis" recorded.
After many years of working untold numbers veterans claims, After winning the first ever VA claim for AO inside CONUS FOR MYSELF. After producing the next three AO claims to win inside CONUS for other veterans. After winning a Korea AO claim for a veteran not serving in a unit on the DMZ. After winning a Thailand AO related sole surviving widow's claim for DIC along with accrued benefits for a non dog handler stationed at Utapao AFB. If I could sum it all up and explain in one sentence the most important fact of all, the most underutilized, hard to comprehend and misunderstood tool in veterans claims it would have to be,
IF YOU CAN GET A DOCTOR TO SAY ITS SO, ITS SO. IF THAT DON'T WORK FOR YOU GET TWO DOCTORS TO SAY THAT ITS SO. IF THAT DON'T WORK, GET THREE!
WHY TAKE A CHANCE, GET THREE TO BEGIN WITH AND NAIL IT THE FIRST TIME AROUND.
HOW CAN A VETERAN AFFORD TO DO THAT ON A TIGHT BUDGET? I PLAN TO DISCUSS THIS EXACT IMPORTANT TOPIC ON THE PODCAST COMING UP ON THURSDAY EVENING OCTOBER 27TH. 6:00 CENTRAL TIME.