I don't think it's necessarily a line-in-the-sand type thing. I think it's possible to have a question here that would make for a good SO question, and then move it over there (new thought for me, as I just posted re. clarifying @garrett yesterday) …
Why not just ask there to begin with?
SO can be intimidating…like, really intimidating. Don't get me wrong, I love it, but sometimes there are questions that you're not sure whether or not it's a repeat/duplicate (especially when you're new to programming and/or R).
My little visual example below (wherein 
indicates, "hooray that's my error message" 
):
