I have always been more on the quiet side, but I wouldn't consider myself anxious. I had a very normal, happy childhood. It's the kids that have been traumatized by some kind of physical, emotional, or sexual abuse that can truly show signs of social anxiety.
Working in the school system for several years, sometimes it is easy to spot those kids who have experienced some kind of trauma in their life. There is something about their behavior and reactions that is different from someone who is just naturally quiet or reserved.