I wonder that about myself also. My diagnosis is curious in that I worked it out myself - I had another issue which doctor's couldn't solve for months, so after a ton of reading around I began to suspect I had diabetes.
One thing I did was get a finger-prick glucose meter. Crucially the other thing I did was took 2 weeks off work, and spent every day walking up and down the stairs 10 times every hour and and doing 25 press ups, while cutting out all sugar and limiting carbs to 30g per meal.
So by the time I visited the GP to tell them I think I'm diabetic, I'd spent 2 weeks drastically lowering my blood sugar. I gather the HbA1c is influenced much more by recent levels than those 3 months ago, and it showed that I was firmly in the diabetic range. I'm pretty sure the HbA1c would have been a lot higher if I hadn't had those 2 weeks, suggesting perhaps I'd had it for a while.
For a few years before that I'd noticed some unusual health complaints (for me), many of them apparently more likely if you are diabetic.