Presumably (?) one can identify which parts have identical dies by looking at the table of quiescent supply current. Parts with the same Iq_vccint:
-12T, -25T (lowest)
-15T, -35T, -50T (middle)
-75T, -100T (high)
-200T (in a class by itself, highest)
Interestingly, for Spartan-7, the two lowest-gate-count devices (-6, -15) have lower Iq_vccint than Artix-7 12T/25T. The higher-gate-count devices (-25, -50, -75, -100) match their Artix-7 counterparts exactly.