There are fundamentally two types of mint marks on US 1979 proof coins. These mint marks are used on all denominations of these coins from the Cent to the Dollar. The type 2 coins are more valuable than the type 1 coins so it is helpful to be able to tell the difference.

It is fairly easy to tell the difference between type 1 and type 2 proof coins from 1979.

The 1979-S Type 2 is nicknamed 'clear S'. You can create this 'S' shape with a clay stick (a round pencil shaped piece of clay). If this stick is shaped into an 'S' without the inner surfaces touching you have the basic shape of the Type 2 mint mark (example below). Now if you refine the two ends to make them bulb shaped you have the 'Clear S, Type 2'.

The 1979-S Type 1 is nicknamed 'filled S'. It can be created by taking the Type 2 clay shape and pressing it together on all 4 sides with a flat stick. This shape is more of a blob than the letter 'S' (example below).

The photos above are just examples. There are a variety of shapes for both mint marks, but given this information you should be able to determine whether the coin is Type 1 or Type 2.