Well, if a member of the opposite sex is seeming to go out their way to be friendly to me then I do think that they are at least flirting with me.
Sure they may just be friendly to everyone but there could be a slight difference in how they are with me in comparison to others, also I think to myself, does this person have a reason to be chatting to me etc, seemingly going out their way to do it? If there's no logical reason for it other than that they like me or are interested in me in a sexual way then i'm going to assume it's the most likely option.