The big ones for me were a frequent, sudden, urgent need to pee and getting up multiple times a night to pee. I also drank a copious amount of water. Like, the whole "eight glasses a day" thing which used to be popular was confusing to me, as I'd drink that much in the first couple hours of the day. I finally went in to the doctor and got a blood test and my A1Cs were well over the "welcome to Diabetes Land" number. With diet, exercise and drugs I'm well controlled now and caught it early enough that I still have good feeling in my feet. Given my family history, and all the shit I ate in my younger days, it's not really a surprise. I just have to be more careful now, but I have discovered an enjoyment of climbing because of it.
Really, if you have any family history of diabetes, start visiting your doctor on an annual basis and getting a blood test. It's simple, and catching it earlier is good for preventing problems with neuropathy in your feet.