....
Scientific fact is:
it is not possible to visually check cure doneness...
..... let facts rule...
I have done what BearCarver says... cut a pork loin to check for full cure penetration, and indeed, it is very easy to see if the center is still not cured. In pork, it is gray(ish).
I also cured some beef Eye of Round, and one piece did not cure to center... and it too was easy to see the difference when first cutting into it.
Scientific? No.
But more scientific than Greta!