I guess it all depends on what is considered fallen, whether it is just a dark angel that happened to be with luthor on caliban before or during the lion's raid, or someone who fully accepted chaos knowingly. I guess the lack of published fluff and details somewhat makes you have to reach into the hypothetical realm, but I highly doubt that someone still loyal to the imperium in thought, purpose and deed would have any exterior markings of any kind save self inflicted tattooing or branding, not including battle scars. I guess my opinion is that chaos would have to have some kind of footing on the dark angel before he would be considered truly fallen, and actually manifest some symbol or mutation due to it.
just MHO...
just MHO...