In the song "Could well be in" by The Streets  it talks about a girl
being "keen" is it slang from where they are from becasue in its
literal sense it doesn't make and sense..

"  I saw this thing on ITV the other week 
Said that if she played with her hair, she?s probably keen 
She?s playing with her hair well regularly 
So I reckon I could well be in  "

How does being keen relate to him being "in" with her. 

Thanks.

-Bryan

If a girl is playing with her hair while talking to you, the girl is
probably keen on you (meaning that she likes you).

Hi Bryan,

Here's how I see it:

"Said that if she played with her hair, she?s probably keen" 
She said that if she played with her hair it would mean she is interested in me.

"She?s playing with her hair well regularly"
She's often plays with her hair. 

"So I reckon I could well be in"
So I guess that means she's interested in me.

I hope that helps.

Rainbow~