Does Dark Skin Mark On Neck Indicate Sign Of Diabetes?

Hi my name is Vince Hernandez and I know I am over weight, but I am not sure if I m diabetic. I m afraid to even find out. I m 5 4 and 33 years old. I have been told that my weight should be around 160 lbs so that makes me obese since I weigh around 215 lbs right ? My question is I have a dark skin mark on my neck that appeared suddenly. Some one had told me that it could be a sign of sugar diabetes, is this true?