Interpretation of possible-flirting is a minefield, it's true. People are highly various in how freely they relate, behave, and flirt, etc. And equally various on what they feel is 'okay'. There's little clarity to be had here.
Were it me, I'd assume it's flirting, or at least that the other person is receptive to that kind of talk & behavior, up to a certain limited point. This is where one really needs to have an idea of what is and is not acceptable though. Thus, a minefield.
The other way of thinking about it is to ignore it unless it's overt. If you consider the prior viewpoint a mistake, then this one is equally so, as many people flirt subtly and expect the other person to pick up on it. So if one does not act on subtle flirting, the other person will assume lack of interest and move on/close off.
Flirting just might be the most subtle and delicate social behavior in existence. ><
However, there's ways to cushion potential misunderstandings... You'll notice some people 'get away with' stuff that others wouldn't. It's because of the personality they've come to exude. People are led to expect a kind of behavior from the person, and somehow end up more accepting of it. And they know how to play it off, for example by not acknowledging that they did something wrong (continuing to smile and mess around despite being rebuked, etc).
In contrast, there's the kind of person who apologizes for everything they do, has trouble looking people in the eye, who easily absorbs the emotions implied upon them by others(feeling guilty when others exude anger toward them, etc). Those people have a harder time flirting, and can't 'get away with' much. That inability to flirt easily is one of the biggest disadvantages introverted people tend to often possess.... Sad but true...
On a side note, I think waiting to find out whether the girl has a boyfriend is a bad idea. It's an excuse to hesitate, which we introverts tend to grossly abuse. Furthermore, if she has a boyfriend who you do not know and aren't sure exists, it is irrelevant. If things progress romantically, then it means either:
A) her existing relationship isn't very good, and should probably end,
B) she has little self-control, or little care for others (And you'll figure that out just about immediately)
C) shes poly-amorous,
D) she does not believe she has a boyfriend.
A couple years ago, I used to be of the mindset that women with a boyfriend were off limits, but I changed my mind. Relationships are meant to fail, most of the time. It's like the scientific method, where theories must be poked and prodded, and the ones that hold true are the ones that resist all assault. The ones that falter beneath it should be let go of.
That said, if I know both people, that would constitute eating where you s**t. So that's obviously a no-go.