I have a barrel that I have fired some corrosive FMJ ammo through, and thought I cleaned it real well within an hour of finishing shooting.  This was about 3 months ago.  I pulled the rifle out last night just to refresh the lube on it (I have not shot it since) and I noticed a pair of dark lines that exactly follow the same side of adjacent lands in the rifling.  After some vigorous brushings and Shooters choice MC7 it seems to have largely gone away, but not completely.

How can I conclusively determine if it's copper fouling or if it's corrosion?

Soak for 30 mins with MC7 if patches come out green it is copper.  If not well I guess you did not clean the rifle as well as you should have.

Water aka H2O is what you use for cleaning corrosive ammo.

As mentioned, Green = Copper. Hoppe's #9 cuts it pretty good too.