The 1970 S small date is one of the most confusing Lincoln Cent varieties for new collectors. The reason for the confusion is simple - it's tough to tell the difference between the large and small date. I have seen 20 - 30 Ebay auctions advertising 1970 S "small dates" with a picture of a 1970 S "large date" cent. When the 1970 S small date was first discovered coin dealers felt that this would never be a popular variety because of the minor difference in design. Certainly its inclusion in most coin albums has added to its popularity immensely.
The easiest way to tell the difference is to look at the 9. If the loop of the 9 bends sharply toward the 7, it is a small date. If the loop of the 9 is angled toward the top of the mintmark, it is a large date. The nine in the small date is also "wider" looking than the 9 in the large date.
Another way to tell the difference is the height of the 7. In the small date the top of the 7 is just about level with the 1,9 and 0. In the large date the top of the 7 is lower than the 1,9 and 0. Beware, this way can be very deceiving.
A third way to tell the difference is by looking at the word LIBERTY. In the small date the word LIBERTY is weak, especially the top of the E and Y. In the large date the word LIBERTY is struck well with no letters blending into the field.
If your only looking at the height of the 7, you may not be able to tell the difference. Try looking at the 9, because it is by far the best way to differentiate the two. Below are some examples:
The 1970 S small date proof and 1970 S large date proof can be distinguished in the same ways. Below are some pictures of the 1970 S small and large date proofs: