I would first have it magnafluxed (MPI)........
Mannnnn, if you're gonna look that closely save the time & money and just toss-it!
I don't disagree with the idea, and probably is the right thing to do, but might just prove an exercise of futility as cracks that would require magnafluxing to spot, if these really were a make or break on whether to reuse or not, with the heat and the smudging of the surface material often these may be hidden below the surface, hence one reason the O.E. just says "toss-it". After grinding won't work either as the grinding stone pushes the surface material, more efficient at removing material but still not so unlike the friction disk does and covers over these smaller cracks. And really the idea that minor surface cracking can be just "ground-out" and effectively returning the piece to "like-new" condition is also a fallacy, though popularly practiced.
Besides, they would all probably fail such an inspection anyway, and then of coarse most customers are under the impression that so long as the fissures aren't too great it'll be O.K.!
So again, there really is no true break-point for yeah or nay for re-usability, just common sense weighing the observation of the current condition, does it appear obviously "done-for" or not; understanding how it got to state that it is in, does the system seem capable or are we "barking up the wrong tree"; and then of coarse direction provided by the 'project engineer'; though realize that for example myself, I have handed customers their stuff back and refused to surface it if I truly thought it was a bad idea.
It doesn't take witnessing to many flywheel explosions to convince one that they don't want anything to do with such an incident!
Scott.