There's a whole science around flirting, and I note that some of the key traits have been mentioned. Also be aware of the potential flirter leaning towards you a bit too much to listen to what you're saying. Often this may be at an angle, so that they can present their best side to you. Nodding a lot and agreeing with everything you say - even laughing at stuff you say in an exaggerated manner. Mimicking your body language is another trait of flirting. I think I can also add the use of suggestive language, double entendres and the potential flirter asking you lots of open questions.
Recent traits of flirting that I have come across, in terms of mobile textual exchanging, include: :-p, xxx, mwah [?], xox. I've also encountered instances of a flirter spending more time with the flirtee than anyone else. Also, the purchase of toffees for the flirtee is something to look out for.