Usually women smile at strangers to appear nice and avoid a confrontation. However, if she does it in a blatant flirting manner that would cause to say something to her in a suave manner to break the ice.
not necessarily, a smile could mean lots of things... but if the person smiles a lot to everyone, then you assume its her 'usual' self. but if she smiles just occasionally, then it might mean that she's interested.
But I think what's most important to know if a girl is interested is her eye contact, you kinda know if she's interested if she stares at you for long time. but this varies among girls.