Context is everything. 99% of the time, i assume she's being nice. The remaining 1% is "this might be a signal..." But, nowhere on the scale is "she's into me!" The best you get is doubt.

Ill give an example. I went for drinks with a friend and the waitress remembered me from past visits on business. After a long night, my friend left me drunk at the table to finish my pint. Waitress ended her shift, and joined me since i was alone with my drink. We had a decent conversation (i think), and it was good fun. Even had a couple more...

If you told me she was hitting on me - id believe you. If you told me she had a tough night, and im a decent conversation (or tipper), id believe that too.

Doubt is the rule.