When Uncle Jesse uncovers his black car in the barn, the driver's door is opened and you can see there is no window frame. Later when Jesse is pulled over by Rosco, the driver's door is opened and it now has a window frame.
When Bo and Luke are arrested along with the female federal agent, as Bo gets out of the General Lee, a roll bar can be seen moving, revealing it to be a fake. A real roll bar would be bolted or welded in solid.