Most Helpful Girl
Its hard to answer a vague question like this! Not all girls are the same, and the situation really is important as well. However if she goes out of her way to talk to you, if she laughs at your jokes, if she makes an excuse to touch you - these are general hints that she's interested and not just being polite.
Most Helpful Guy
I can think of some honest reasons why girls would be being so nice to you and some reasons that are not so good to.