The advice given in that thread is really the only positive way of telling your diff ratio
There is a list of the different diff's fitted to the Sierra in the Haynes manual .... Not really the propper way to ID a diff that may not even be the one originally fitted to the car !!??
Sometimes you can give the ID tag a good clean with some sand paper, if not too corroded you may be able to read the markings.
That said, still not proof that that was the tag originally fitted ( More so if it has been sold as an hard to find low ratio one )
Counting turns/teeth is the only sure way .... Most of the time the ratio markings are still visible on the crown wheel but that would still assume that the diff has never been meddled with.
If you really need to know & none of the advice given, floats your boat, perhaps a visit to a gearbox specialist will put your mind at rest
