How do you know if there is room in your growth plates?

My son had knee x-rays at age 15 and when his orhtopedic doctor examined the x-rays he commented that my son was going to grow much taller. I asked the doctor what he saw on the x-ray that caused him to think my son would grow taller. He told me he could tell by looking at the space between the growth plate and the bones in the x-ray.

My son is now 17.5 years old and is 6'2" and still growing. (My son was only 5'6" at age 15 when he got the x-rays.) My son was always "average" in height compared to friends of the same age during his childhood. He just grew by leaps and bounds once he hit high school.