Look at the tag on the rear end cover If it's all bent up it was probably removed when the gears were changed, if they were. If it doesn't look like it was touched then read the tag. The first 3 numbers on the bottomw row of the tag is the gear ratio, the next numbers are the axle size.
An example of how it would read would be:
"3 08 8 8"
Which would indicate the rear has 3.08's and is an 8.8" rear.