My son was diagnosed when he was 17. What tipped me off was his insatiable thirst. As background, he hasn't been feeling well for a few weeks and had some stomach/GI issues, but nothing really remarkable. One day he said to me -- "Mom, I just can't get enough to drink" and the lightbulb went off in my head. I took him to the pediatrician the next morning, they tested his urine for sugar, and immediately sent him to the ER of our local children's hospital. We both were in the hospital for about four days while we learned how to cope with the disease (he immediately was put on insulin).
I hope and pray that your son doesn't have diabetes, but if he does, it's best to know about it as soon as possible. If I can help in any way, please PM me.