I'd guess this would be pretty simple.
If we are to give it a "pure GNU/Linux" stamp, it must contain Free Software only. So every package on the distribution must qualify according to the Free Software definition of the FSF.
Yes, the principle should be simple and easy to operate.
This of course includes setup programs and such.
Setup program will be one of the focus to check. Another dim area is to check the license of the device drivers inside the kernel. I think packages and applications are easier to check than these two.
Best, Frederic