rear diff

Sometimes you can find a metal tag on the diff telling the ratio. Or you can count the number of teeth on pinion gear and the ring gear and divide the larger number (ring teeth) by the smaller number (pinion teeth). You might get a ratio that looks like 4.10 or 3.55 or 2.73 etcetera.

do you have any idea what the most common ratio is for it?

3.55 and 3.27 are very common. Sometimes the ratio is also stamped on the outer edge of the ring gear. If you're rebuilding with a full ring and pinion kit then you can choose what ratio to run to suit your best. Low numbers are slow to accelerate but with lower revs on the highway, higher numbers give better acceleration at the expense of higher revs on the highway