Usually there is a little silver tag on the back cover attached through one of the bolts if u are lucky.The last 3 numbers on the tag will be the ratio ie 362 or 393 etc.
If not take the rear cover off the diff and it is usually etched into the crown wheel(the large one).
If not the mark one of the side flanges and then rotate the front drive flange until the side flange does one full rotation.Keep note of how many revolutions you do
