None of the reproductions come with the four small holes on each side for the original side curtain studs. there's also a subtle difference in the cross section with the reproductions being slightly flatter.
The repro frames are for deluxe roadsters which have the rear view mirror mounted on the left windshield stantion. Standard roadsters had the rear view mirror mounted in the top center of the painted (not chrome) windshield frame. So a standard roadster/phaeton windshield frame can be identified by the two rear view mirror mounting screw holes. If the frame you have has these two holes chances are it is an original. This is another way to identify a repro vs an original an original windshield frame.
Just the shape of the frame is easy to see. Repops are flat. Look at the picture Chris posted it has more shape. Does anyone make one just like originals?