Used, so the previous owner(s) may have changed out the diff gearset
Best and easiest is to tie a length of string to the U-Joint. Choose the one pointing straight down and roll the vehicle until it is pointing straight down
Roll the vehicle one tire rev. Use the valve stem as a marker
Then count the whole number of string turns. That will the the whole number...3 turns means 3.4 or 3.7...4 turns means 4.1, etc
Then the fraction of a turn will be the rest of the diff ratio
Remove the string (have to say this, as assumed they would in past posts and got some PMs yelling at me that I should have said that)