It wouldn't be too difficult to confirm that the satellite was damaged by an impact. You could detect the rotation rate, orbit change, and other telemetry points to determine that something impacted. It just seems difficult to specifically say it was a piece of space junk that caused the damage. Out at geosynch, the ability to detect small pieces of space junk is extremely limited. If the impacting object was of a detectable size, it seems likely that it would've done too much damage to leave them with the ability to supersynch the satellite. More likely, it would've been destroyed unless the relative motion at impact was very small. If the object was below detectable size, then while it might've been space junk, it might've also been something natural like a micrometeroid. However, you probably wouldn't be able to confirm it one way or another.