How do you know if the 1979 dollar coin is a type one or two?

The type 1 and type 2 1979 dollar coins pertain only to proof coins and refers to the mint mark. The mint mark will always be an "S". The type 1 mint mark is called a "filled" mark because the "S" has a smaller bottom curve than the top curve and lacks detail giving it a "filled in look. The type 2 is a more perfectly shaped "S" with the top and bottom curves being more in balance and the mark has far greater detail than the type 1.