I am scared as s*** when it comes to beautiful girls. I'm scared of denial, or worse, being laughed at. For me, and for most guys, being intimidated will be the norm.
If you happen to be one of those "extremely attractive girls", just be generally kind and approachable, or try to downplay your looks slightly. Another association guys make with beauty is snobby, stuck up, or a general b****. So proving that association wrong is your best bet.
Sadly, most guys in America will be too intimidated. Not talking about the bar where everyone is drunk.
The only sign will be that they will keep checking you out but that's about it. Signs? Perhaps their palms will be sweaty... their body will be stiff. Simply put it, they will look uncomfortable which you girls will probably think creepy.