In 1970, there were two Master Dies used on the obverse of the Lincoln cent 1¢. Strangely, only a few dies were created using the so called Small Date, and all went to San Fransisco. There are an assortment of misguided ideas about how to ascertain a genuine Small Date from a Large Date. The simplest, and surest way, is to look at the 9. While the height of the 7 is a factor, as is the weakness in LIBERTY, the 9 is a completely different style.
As my illustration below shows on a 1970-S, the inner loop of the 9 should POINT to the middle of the 7 on a Small Date.
The inner loop of the 9 will POINT to the bottom of the 7 on a Large Date. Sometimes Large Dates will have a weak, or abraded LIBERTY - they may also show weakness in the date and make the 7 appear higher. Always check the 9. There are no very high or very low 7s as is sometimes suggested. The style of the letters changes between the two Master Dies - the 9 is most noticeable. Only buy a Small Date when the seller includes a picture large enough to see and assess the 9. Good luck.