1968-1969 had round reflectors instead of the squarish ones on the sides. So, we know it's not that old.
Like certdubtech said, in 1973 they moved the front turn signals from below the headlights to above the headlights, so we know it's older than 1973.
In 1972, they went to the larger, rectangular tail lights (with integrated backup lights), which narrows the year to 1970 or 1971.
I don't know of any other visual differences between a 1970 or 1971.