Most Helpful Girl
Oh hahah so many things (personally). It does really depend on her personality though.
A shyer girl may not give many clear hints that she's into you. Sometimes blushing, small smiles, aiming to be near you as often as possible (when you don't go up to her) are signs. If she consistently jumps at opportunities to hang out/work with you that's a pretty positive sign. On the other hand, it's much more efficient and clear to just ask her friends to ask her. We normally love to go on secret missions for men interested in our friends!
A girl who is less shy will seek you out to talk to. Make a lot of jokes around you, smile (with her eyes... that's key) and laugh at your jokes, sometimes even if they're subpar. She'll flirt with you, perhaps invite you to hang out or do something with her. Physical contact, grabbing your hand, leaning into you, hugging are all good signs.1