Hello Ibam,
it's in the colors and it's in the bokeh.
Both lenses have those abundantly and the Flek is really versatile.
This is the Flek uncropped, nopostprocess, full open,
This is the Sonnar, suffering from bad postprocess and compression, colors still there,
And yes you're right the aperture housing is the same. As an extension to the before-linked disassembly tutorial you can have a look on my flickr for the Sonnar disassembly. As a last resort you can completely disassemble the aperture if it's stuck - not recommended, better clean with alcohol from the outside til it snaps again.
Serial numbers are not import / export oriented. The later ones have the smaller numbers. Other than that I've not seen any means to distinguish image quality other than taking oh-so beautiful pictures with them :-) BTW, there's not the old '1Q' sign on any of these, that was only on older production.
More Flek pics here,
Flickr: The CARL ZEISS JENA DDR FLEKTOGON 35mm f/2.4 Pool
Good luck in finding one of these and nice pics,
Georg