Even within the "safe" confines of my TG group meeting I wouldn't dare make any assumptions of anyones birth gender. We have both MTFs and FTMs. With some you'd have no idea at all, except maybe to be 180 degrees out of sync with reality. Forget about even thinking that out on the street with total strangers you stand a half chance of reading a TG and being absolutely sure. In fact, back in the late 70's it was'nt rare in NYC for a GG to say she was a trans, just to blow off a guy hitting on her. You just cannot know for sure.
As others have noted, the LAST thing most of us want is acknowledgement that we were read! By ANYONE! (and that includes ourselves). I settle for "you look OK" when I look in the mirror and cherrish the days when I can say "You look F'n GREAT!" before I go out, or better yet when I return home. The other times I change my outfit or redo my makeup.
A small aside - My wife (30 years post-op) and myself often wondered about Lady Gaga, simply for the fact that she looked too perfect. Conversly, Madonna, IMHO, tends to look like an old drag queen these days when not performing.
Caveat Emptor