If a guy is talking to you, he is into you (or wants to be)

Unlike most women in the universe, guys generally don't make conversation out of pity. If they are not interested in a lady, they usually make a beeline for her cute friend and ignore her completely.
It's not in their nature to be friendly, nice, or super engaging in conversation like most women feel pressured to be with everybody (oh how I envy dudes for this very reason). In fact, I don't even think they care that much about feelings; they care about getting things done. If talking to you gets them closer to their goal, they will indeed do so. Otherwise, they really don't give a shit.

I think every woman secretly knows this but also enjoys playing the denial game in order to acquire "more friends" or simply abate their boredom at work. Women love it when people listen to to them talk and latch on quickly when they find a willing and very eager ear to gab to. This surface level denial seems to cloud rational judgment and understanding of the true state of things. Even if the man has a wife, long-term girlfriend, long-term partnership, whatever it is-- if he is initiating conversation with you on a regular basis and consistently entering your stratosphere, chances are that there is at least a small portion of his bastardly self that desires to bed you. This may sound vulgar, maybe even shocking, but I think just about every male on the planet does this. For a very viral representation of what I'm talking about, please visit this video about why guys and girls can't be friends:

So now that you know the truth and you admit it, how do you respond?
If said dude is unattached and someone you are potentially interested in, you could respond in one of two ways to be effective (time-tested methods that drive men crazy):

1) Be aggressively flirtacious back
2) Be passively naive and continue to project "unawareness"

Depending on your personality, choose the style that works for you. Since I am shy and dorky, I find that playing the innocence card works quite nicely.  This tends to attract aggresssive men who want to protect me. On the other spectrum, there are men out there who really appreciate someone with sass and attitude. So break out the touchy feely and flirt back shamelessly if that is the way you are wired.

If a guy is talking to you, he is into you (or wants to be). End of story.