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Simplest is to put it in unity gain feedback with the + input somewhere in the valid common mode range and take the difference between inputs. I like a vcvs A=1 to get a node voltage that's real time valid. You might find it interesting to look at Vio across the common mode range, because how that plays might end up determining the ICMR you would advertise.
Better (but much slower to run) would be a classical op amp test loop. This has been posted here before.