Short
answer: You don't.
Slightly longer answer: My partner calls it "vibes"--
something you feel. But he's from California and
Californians think everything has to do with vibes.
Perhaps it has something to do with living that close
to the San Andreas fault.
There's something to be said for vibes, but sometimes
you just can't tell. I think you may need to be
a little more direct. If you think he's flirting
with you, why not ask, "Are you flirting with me?"
