Instinctively, you know that a guy's facial expressions, gestures, and mannerisms are windows into his mind and emotional state.
On a good date, you'll have surefire proof he's into you, and on a less-than-great date, you'll be saved from potential embarrassment or even clued into the fact that he's not worth a second drink.
When you were around 10 years old, chances are someone passed a folded sheet of notebook paper and it read in 10-year-old scribble: "Do you like me? Check yes, no or maybe." You smiled and checked the "yes" box. But as we've gotten older, the dating scene's gotten much trickier.
Nobody passes notes with yes, no or maybe boxes anymore. There are other ways to make your intentions known, or to know when someone is checking you out, and flirting is one of them.
If someone instructed you to "Go over there and flirt with that guy," you'd inherently know what to do.
You'd communicate through a series of body language signals, both conscious and subconscious, that you're interested and open to his advances.