Here's a simple way if you don't have your Diff torn apart yet.
Jack one wheel off the ground and count the number of drive shaft rotations it takes to turn the tire one full rotation. Multiply this number by two and that is your ratio. If you have a Posi or limited slip differential, you may find it difficult to rotate the tire by hand. If this is the case lift both tires off the ground and support the vehicle with jack stands to count rotations for one rotation of both tires. This isn't as accurate but will get you close to your ratio.