I am considered overweight because a doctor calculated my BMI and said I'm two kilos over where I should be. BMI is a the ratio between weight and height. I think it's kind of stupid but it got me labeled as overweight.
It also applies less as you pass 2 standard deviations of average height.
So, if you are 6'1 and have bulky shoulders and no belly, your bmi should actually be on the high side. Warning signs are when you see ribs or when you can pinch belly fat.