Ron Guth: 1970-S Cents are found with either a Small Date or a Large Date. Both varieties are common, but collectors like to have one of each. To tell the two varieties apart, draw an imaginary line that connects the top of the 9 and the 0 of the date and another connecting the bottom of the 1 and the bottom of the 0. On the Large Date, the top of the 7 will be below the top line and the tip of the 7 will below the lower line. On the Small Date, the top of the 7 appears to touch the top line and will not go below the lower line. Both varieties are readily available in Mint State, though only a few have been certified at the MS67 level and none have been graded higher.
Rick Snow offered a great tip for telling the two varieties apart: the word LIBERTY is weak on the Small Date but strong on the Large Date.