With the green line sticking out of it, it's easy to detect which direction a normal is facing. If that part of your model used the color set in the configuration for a back-facing polygon, it's utterly impossible to tell if it's correct or not. With the green line sticking out, it's easy to tell no matter what color your model's texture uses. Otherwise you have to either disable textured mode, or uncheck the "draw backfaces" option from the menu and at times, it can be tricky to find this.