The differential protection relay
There are two basic types of CT saturation. AC saturation which is simply due to the burden presented to the CT being too much. AC saturation needs to be identified and accommodated in the relay settings and design up front.  The second way to saturate is DC saturation which occurs when the CT is presented with a primary current that has a DC offset - IEEE standards address how to analyze the CT performance to these factors.

That said, I believe the question you are getting to concerns ensuring the differential relay is secure for through faults in which DC offset in the primary current waveforms can result in operate current that trips the relay.  Years ago at GE, Bogdan Kasztenny and others developed a method that recognizes that the operate current is due to DC saturation.  They take advantage of the fact that DC saturation of CT takes time to occur, it does not occur immediately like AC saturation.  So, when they see high restraint currents like you see during external faults...and THEN after a short time the operate current rises they can surmise that that operate current showing up was due to CT saturation. With internal faults, both restraint current and operate current will go up at the same time.  I believe GE and SEL relays use this approach nowadays.  Find GE T60 instruction manual or SEL 487 instruction manual.  Google Bogdan and you can find his papers on this topic.

Russ