If someone is genuinely interested in you and what you have to say, their body language will be open and tend to mirror yours - after a couple of minutes of conversation, if you change position to lean against a wall, or put one foot slightly forwards, if she follows you it is a sign that she is genuinely in tune with you.
Someone who is forcing themselves to speak to you and isn't really interested in what you say will keep up a position that is more defensive, such as arms crossed in front as a barrier, body/feet turned away from you, etc. Looking you in the eye, responding to what you say with appropriate expressions show that she is listening to what you say.
Make sure that she isn't just nervous if she does cross her arms/look away a lot, she might be really interested but likes you so much that she gets a bit fidgety - this will be easy to distinguish from disinterest.
Making eye contact for a second or two, then looking away, then making contact again, is often a subconscious flirting move.
The large grin when you meet is already a good sign!
From what I can tell you might be your own worst enemy here - you are naturally pessimistic, and if she feels when she talks to you that you don't trust her reasons and are constantly trying to analyse her or figure her out, then regardless of whatever prompted her to start talking, she'll become uncomfortable when talking to you and she'll stop.
Bottom line - she's a human being, just like you are, and even if she has started talking to you out of pity, the first contact (often the hardest thing to do) has been made, and from this point on nothing is set in stone - you are more than capable of becoming a good friend to her, and from then on anything can happen, and you'll still have a good friend at the end of it all.
If she does pity you, change her mind! Show her that you are fun to be with, easy to talk to, and not someone who needs pitying. If this is the reason she started talking to you, then she is a good person who wants to help others, and a good ally to have in this world, whether or not you end up together romantically.