I suspect this isn't teachable.
I come from generations of flirts. I think it came along with my eye colour and tough fingernails.
If Setanta isn't flirting (with women, babies, dogs, plants, bugs), I'll know it's time to bury him. I'm pretty much the same. It's like breathing.
Truthfully, I don't much care whether people know it's flirting or think I'm actually interested in them. I'll clarify it for 'em if I have to