I actually believe flirting has become a lost art. In fact people have moved away from subtleties when it comes to showing interest in one another. Everyone these days seem to prefer a "direct approach" when it comes to (communicating) one's interest. The days of looking for "signs" such as a woman flipping her hair, batting her eyes, or crossing her legs in a man's direction often times doesn't mean she's interested in him. Too many times misreading signals leads to embarassment or hurt feelings.
Today it's all about observing the other person (after you have said hello) and engaged in a conversation. Does she or he smile frequently while talking to you, listen intently, appears engaged in the conversation, offers up their opinion on the subject matter "enthusiastically", laughs at your humor or wit, and agree with your line of reasoning. These are all (obvious ways) of knowing someone is "agreeable" however it still does not tell you whether or not they see you as "relationship material" or platonic friend. Ultimately someone has to make a bold move of asking the other out for a date or leaning in for kiss...etc
One of the reasons we want to be able to "read signs" or let someone know we are interested without words being is exchanged is because none of us enjoys dealing with rejection. If you flirt or use sexual innuendo with someone who becomes offended you can always lay claim that you were (only playing) and say, "I can't believe you thought I was serious!" Awhile back I wrote a hub on dealing with this regarding pheromones. All told I believe today people want to know up front if you are interested in them. No "Jedi mind tricks", "hints", or asking friends to find out if someone likes you...etc "Freedom lies in being bold" - Robert Frost