Either the numbers stamped on the drivers side pan rail or the VIN stamped in the raised pad on the passenger side bellhousing will tell the model year. The VIN will tell if the trans is original to the car.

Another quick way to tell '71 from earlier is the middle pipe plug on the passenger side of the case, if the plug is located down low near the pan rail it's '70 or earlier, '71 and later plug is higher up on the case. Pic below is '70 and earlier.