Best way to know is jack up the back and count how many times the wheel spins for 1 driveshaft spin
Actually it's count how many times the driveshaft spins for one wheel revolution isn't it? The driveshaft would spin basically 3 times around exactly for 3.07:1 gears. And basically 4 times around for 4.10:1 gearing. 3 1/2 times for 3.55:1 gearing. Etc Etc.
Look for the steel data tag on the diff cover which has the BOM number and gear ratio stamped in it. Typically on the left side of the cover. Assuming it's completely stock and hasn't been modified or re-gear in the past.