Is your question regarding whether it is an original GT40 ZF, or to verify whether it has been flipped and suitable for use in a GT40?
There should be another digit after the 5DS-25, a -0, -1 or -2, unfortunately most later -2 flipped ZF's don't identify any differently, unless the person took the trouble to identify somewhere. You can verify by rotating the output shafts together in a forward motion and noting which way[[the input shaft turns ]] with a forward gear selected.