Ok, go up to her, & say, "Hi, please don't think this is a
chatup line, because it isn't. If you tell me No at the end of my
question, I won't bother you again."
Look at her body language & judge if you can proceed. If yes,
then tell her, "I'm actually chatting with this girl now, & I
think you might be her. My nick is blah blah blah, & her nick
is blah blah blah. So, am I right?"
Like that lorr 
FeowFeow