Quoting ~Julie Blue Eyes~:" To me, flirting is never "innocent" What's the point if it's not a means to an end? You flirt with people you are attracted to and/or interested in."
flirting can be absolutely innocent.
i flirt with everyone that i like, and i'm not even bi-sexual!
regarding the OP: if you know it's not innocent, it's not. we all know when it's not - unless we're somehow deaf/blind or otherwise unable to read all the cues that make it instantly clear when a woman or man is no longer just being friendly in their flirtation.
there's no set behavior, it can be as simple as glance up and down the body or
as complex as a FB interaction.