Likewise I can tell that English is not your mother tongue.
I understood everything you wrote (you're obviously fluent), but tiny little things give away that English isn't your native language:
> learner's with native tongue that hasn't these features
I would have written: learners whose native tongue doesn't have these features
> but we just let it slip
but we just let it slide
> you've not grown up here
you didn't grow up here
I sincerely hope you don't take my comment the wrong way. It was too interesting to pass up an example of the very thing you mentioned: that you can tell even when the person is fluent (even if you don't hear an accent).
Related observation: You know how Ted Kaczynski, the Unabomber[1], was caught? He used the expression "You can't eat your cake and have it, too."[2] But nearly everyone knows the expression as "You can't have your cake and eat it, too." Ted Kaczynski's brother recognized Ted's highly unusual usage from the anonymous manifesto published by the Unabomber, and this is what convinced him that he was the Unabomber.
[1] https://en.wikipedia.org/wiki/Ted_Kaczynski
[2] http://itre.cis.upenn.edu/~myl/languagelog/archives/002762.h...