Depends on the guy. When I (being someone who UNDERSTANDS body language) is "testing" a girl to see if she "bites", I keep eye contact, and break it slowly (generally while smiling), I will engage in small talk, I'll do things to make her comfortable around me like introduce her to friends/aquantences, I will touch her if she makes me laugh etc... I give her a wink or two when appropriate... Give them my classic sh*t eating grin (haha).
I've yet to have a girl that DIDN'T know I was flirting with them when I was. Oftentimes I have girls thinking I'm flirting with them when I'm not... (I tend to be a flirty (with women) alpha male type...)
It really depends on the guy. Some guys (READ: MOST GUYS) have this ego where they "have to figure it out" on their own without spending a few weeks studying body language, flirting etc to become more attractive to themselves and others... That's fine with me, it neuters my competition.
Women usually have a very good (READ: BETTER THAN MEN'S) intuition when it comes to attraction. So trust your gut.