they are nice to everyone because they want to be loved by everyone, or they want to make everyone feel good, i have a friend that used to date a girl like that, you can tell 'initial hints' if she talks to you alot, any girl that is nice to everyone is just the same as others, if they are interested in you, they will talk more to you.
If she remains to be nice to everyone, then that's it, she isn't interested; they tend to be good talkers, so try to have interesting conversations with her... Don't behave like the others and use her to hear your problems, don't request her pity, she will see you aren't using her as the others use her, and she will most likely talk to you about her own problems.
The friend i mentioned earlier though he was dating this very nice girl, but through out the relationship she turned to be a profoundly depressed girl with low self esteem. It's cool to meet a nice girl, but remember that that's not all they are...
Most nice girls are shy, so you have to make the move later on.