How do guys know when a girl is interested vs not interested?

I'm asking strictly from the man's point of view. Some things men would consider to be positive or negative signs.

Lets say a guy flirts with a girl regularly. Without coming straight out and saying it,

What are some things the girl would say or do that would make the guy think she's interested? That would encourage him to continue with his flirty behavior.

What are some things a girl would say or do that would make the guy think she's not interested? That would discourage him from flirting in the future.

How do guys know when a girl is interested vs not interested?
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