Well, that's the whole idea of flirting - that you can't really tell the difference. If it's clear and upfront, then it's not called flirting anymore, but rather an advance (friendly or more explicit).
You have a lot of uncertainty arising from a simple gesture/look/invitation, and (I believe) this is where all the fun really comes from: dealing with a lot of different scenarios that have very similar initial contexts but have a wide range of possible outcomes, and choosing the outcome you want with so little effort.
I also believe that your ability to tell the difference between one person's flirting and friendliness is strongly influenced by how well you know that person.
Well, that's the whole idea of flirting - that you can't really tell the difference. If it's clear and upfront, then it's not called flirting anymore, but rather an advance (friendly or more explicit).
You have a lot of uncertainty arising from a simple gesture/look/invitation, and (I believe) this is where all the fun really comes from: dealing with a lot of different scenarios that have very similar initial contexts but have a wide range of possible outcomes, and choosing the outcome you want with so little effort.
I also believe that your ability to tell the difference between one person's flirting and friendliness is strongly influenced by how well you know that person.