A girl stringing you along will be "hot and cold": she'll tease you sexually and then act like she doesn't want anything to do with you. She'll also try and manipulate you in some way so that you can keep your interest in her: like saying that she has plenty of other guys who want her, but she chose to be with you.
I don't think there's such a thing as "playing hard to get". Some women just are. They don't know your intentions with her, so they're cool to you until they get to know you better, slow to warm up.
Its easy if she is playful and rejects you once or twice and then she agrees to go out with you or do anything with you then she's just playing hard to get , or if she seems uninterested but at the same time she gives you attention and she agrees to go out with you then she's interested.
However if always has an excuse , if she just knows who you are when she needs something , if she gives you no attention whatsoever yet doesn't like it when you dont give her attention and she refuses to do anything with you its safe to assume that she just wants you there for support but not for an actual relatonship.
Playing hard to get is when the person is into you but makes you work for it/them and they'll eventually cave in.
Stringing someone along is pretty obvious though they may flirt with you occasionally they don't show much interest in you, won't give you the time of day, uses you to their advantage, and clearly has or shows interest in other people and more than you but you're blind sighted because of how much you want them... Over time you can tell because it will always remain the same.
If she's stringing you along, then she will likely maintain a physical barrier (e. g. no kissing, platonic lean-in hugs). Bottom line is both have the same response. You are not dating exclusively. See other people and don't let her become your focus (even if you really like her). If she's interested in you, she will likely start initiating more contact (such as touching your arm for more than a second), she will even chase you a bit. If you keep chasing her while she is "playing hard to get", then it will definitely turn into stringing you along until she finds a guy worthy of giving her attention to.