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?

Use a copper solvent. If it's copper fouling it'll come out green. If it's rust you should have already seen orange/red rust when you recleaned it.

Even if it's rusted, if you can't see pitting you're ok. A little bit of steel makes a whole bunch of rust.

What ammo? What rifle? How did you clean it originally?

BSW

Thanks for the response.

It's a WASR-10 that I fired about 100 rounds of early 1980's Yugo mil-surp (1260 round wooden crate sealed up like a danish ham from AIM surplus) through.  I have about a 1/2 hour commute from range to home (don't recall the wx, but it's an indoor range so RH would be the only factor) and immediately cleaned the weapon.  I started with lots of very hot water down the barrel, gas tube, and inner receiver followed by a very through drying.  Following that, I did my routine cleaning which is Hoppes # 9, patches and brushes followed by a light coating of all wear surfaces and the barrel with Breakfree LP (not CLP)

When I did the initial cleaning I did not have a flashlight that I could fit into the receiver while looking down the muzzle to verify cleanliness.  I just kept "looking at the paper after I wiped" to borrow a phrase from another thread here.  I recently bought a single AAA cell mini maglight that I can shine up the barrel when I noticed the thin lines hugging the two lands.

Last night I applied lots of shooters choice and some vigorous brushing.  At some point during this process, I thought the lines looked like shiny copper wire (wishful thinking or truth-I dunno) but now they are a bit dull and dark.  My patches came out nice and green (like verdigris) but I don't know if that was barrel copper fouling or from the brush bristles (doesn't bronze have some copper in it?) so I can't be certain about the nature of my skidmarks.

It seems like the S.C and brushing helped matters but I can't seem to get rid of all of it.  There does not seem to be any pitting.

You should be fine. I wouldn't worry. The only thing I would do in additional is to clean and check for corrosion a couple of days after shooting in addition to the immediate cleaning.

You can read the section of the manual that deals with cleaning: http://www.box.net/shared/cu2djae1zb

Just substitute your favorite water based cleaning solution for the 'alkali solution' the manual calls for. BSW