Back lash is measured with a dial indicator and you'll need a means to hold it,like a magnetic base. There's some other concerns when setting up gears.You need to adjust preload on the pinion and carrier bearing. A sensitive torque wrench with a range of about 30 inch/ponds or a spring tension scale is used to measure the preload. Also pinion height must be checked/adjusted. Marking compound is placed on the gear teeth and after a few rotations the pattern is read.
This will tell you if the pinion height is correct.
As you can see there quite a bit to properly setting-up a third member. If you're serious about doing it yourself I suggest you get the Suzuki FSM. It's the best source of info for this job.