Hi Marina,
Lovely avatar BTW.
I am of the mindset that I would not approach a fellow CDer while she is "en femme" in public. While I would not have a problem with a sister CDer coming up to me and saying you look good (I do not pass so I hold no illusions that I am fooling anyone), I am not sure if all feel that way. Some are trying to blend and going up to them "no matter how innocent it is, could throw them off and upset them (it really depends on the person). Besides, I would be mortified if I approached who I thought was a CDer and said you look great only to discover she was a GG, it would look a bit creepy and if her boyfriend or husband happened to be close at hand . . . it might get ugly . . . "What are doing hitting on my wife?"
Hugs
Isha