Only way to tell for sure is to pull the cover and count the bolts in the ring gear. However, most 10 bolt rears have 10 bolts in the cover, and the same for 12 bolts. So you can just glance under the car and count the bolts on the cover and that's probably correct. A 12 bolt will have 2 bolts on the bottom, parallel with the ground, if that makes sense, and a 10 bolt has only one bolt at the bottom of the cover, so it's real easy to tell the difference once you know what to look for.
If you look under your car from the back and there are two bolts on the bottom next to each other ( the same distance from the ground) then it's a twelve bolt. I don't know if you know what I mean but a ten bolt will be such that only one bolt will be at the lowest point on the rear.