Sadly, despite the wishes of those who go 'out' en femme, 99% are pretty easily read. The remaining 1% often also have visual give aways that signal to look at other items to verify, such as big hands, big feet, broad shoulders, facial shape, the list goes on and on. Voice is another obvious clue, as unconscious clearing of one's throat often comes out in a different tone than the voice. So it's usually pretty easy to spot someone even if you're not looking for it.