I'm mildly germophobic, but since alcohol kills germs, I'm not as fussy about sharing an alcoholic beverage as I would be about sharing something else with someone I just met.
Seems like everyone I've hung out with in the last 10-15 years (since I moved back to CO) is in the habit of sharing/tasting whatever else anyone at the table might have, especially if it's out of the ordinary (not just drinks, but food, too).
I don't recall this practice being that common where I was born and raised, and sometimes newcomers to the group (since I've been in CO) are a little taken aback at the idea, so it's obviously not universal.
So......kinda hard to say if it was flirting or not. I almost feel like *offering* a taste would be more flirtatious than asking for a taste. Or either could just be friendliness or curiosity.....