Is it flirting, or are they just friendly?

The difference between flirting and friendliness is never the words themselves. It is whether the same words get spent on everybody else in the room.

How to read it

Flirting is friendliness pointed in a direction. The tell is exclusivity rather than intensity: a warm person who teases everyone is simply being themselves, and a reserved person who teases only you has told you something they have not said.

The second tell is that it moves. Real flirting gets slightly more personal and slightly more private over a few weeks. Friendliness stays precisely where it is forever, because it has nowhere it wants to go.

When it means nothing

Some people flirt as a reflex, particularly in groups, at work, or after a drink. If it never survives into a one-to-one conversation, treat it as a habit rather than a message meant for you.

Your move

Flirt back once, clearly, and watch what comes back. Ambiguity takes two people, and somebody has to be the one who stops playing.