How do you know if a woman is flirting with you?

I grabbed a breakfast sandwich on the way to work today. The woman behind the cash register wrote her name on the top of the box and wrote "Have a wonderful day" on the inside of the box. She also delivered the sandwich to my table after claiming the pager she had given me was defective. Was she flirting with me or just great customer service?
How do you know if a woman is flirting with you?
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