When both the performance and appearance of the cell (down to using a 10x loupe to compare the top cap shape and metal thickness, top cap seal material, top cap crimp dimensions and tooling marks, top cap metal surface finish, can tooling marks, venting disk configuration and ID marks) match a known Samsung/Sony/LG/Panasonic/Sanyo cell I am pretty certain of which cell is being rewrapped. The cell grade can easily be estimated by any slight shift on capacity or internal resistance as the shape of the discharge curve is still the same.
Can I prove which cell it is? It depends on whether performance and appearance is enough.
I haven't seen a rewrap worse than B grade in over a year. To compare to that, sometimed the only OEM cells available from the vendors we use are B grade Samsung/Sony/LG cells. I feel this is what's been happening with the LG HG2 this past year. The internal resistance is a bit higher and the capacity a bit lower than the cells we had access to last year. There's only a small difference between A grade and B grade cells.
While each of us has to decide which cells we are and aren't going to use, we're stuck when it comes to non-18650 cells as there are none made by Samsung/Sony/LG. If we use any non-18650, we're stuck with unknown cells (except for the Basen, Brillipower, Efest 4200's). I can respect that makes using non-18650's a no-go for some.