Who determines if a jury is "impartial".
Nobody is doing it now. That's the point. A long time ago judges routinely did their best to inform the jurors that they were to be impartial and what that meant. In doing that judges upheld the Sixth Amendment.
Where do you get impartial juries?
Again, it's not being done today and that's the point. The jury is selected from a jury pool -- that's still how it's done. What isn't being done is the judge aren't giving the jurors instructions that include adherence to the Sixth Amendment.