Is having high blood sugar levels a sign of diabetes?

Yes : If you are running consistently high blood sugars (>100-110), then you are at the very least on the verge of diabetes. High sugars are caused by either not enough Insulin (type 1 diabetes) or not being able to properly use Insulin (type 2 diabetes).

High blood sugars: Are = diabetes: fasting sugar 100-125 is pre-diabetes and > 2-hrs after eating 140-199 is prediabetes. Fasting 126 and higher and > 2 hours after eating 200 and greater is diabetes.