Jack up one wheel and turn it and count the rotation of the prop, then halve it. Check for oil fillers in the diff pans, Series have the filler in the diff not the axle, Range Rover diffs often have no filler as its in the axle. You can get a diff pan with a filler for about £25 but it has to be welded on. It is possible to machine a filled into a later diff but of course you need to do this before its fitted. I run 3.54 diffs in my series and like them because mine is converted to a camper so we do a lot of trip over 100 miles at a time. BUT, 1st and reverse are very high, and the 3-4th gaps is very wide. I get around that by using the OD in 3rd t split the gap and reversing in low range, but even then tickover is a brisk walk. Towing I pull away in low range.
