A sure body language signal of flirting is beginning to “publicly undress”. This can be as simple as loosening the tie and collar, or in this guy’s case, undoing his shirt sleeve. Although they appear to be just about to kiss, note that the girl is hanging on to the railing and not to him. This guy may be more sexually attracted than the girl. He is drawing her in close around the waist, but her pelvis is pulling back.