How can you tell if a naturally flirty girl likes you or not?

You can easily tell by testing her out by saying: 
"Look honey, I'm from Tyneside [don't mention Sunderland FC of course]; and she replies:
"oh my god!--that would be my heaven on earth..!!!"
---Then you know you've certainly cracked it.

You ask her=)

Body language, Eye Contact, and just noticing how Comfortable she feel around you...

Well for one thing if she wants to be around you a lot and is always looking for you in a way.. Like at school or something. Also she may make an effort to talk to you moree :)
oh ummm also.... Shell wheel you more than she does other guys