Most yarn is composed of combined fiber strands called "plys". To make a cohesive piece of yarn these plys are twisted together. However there are two directions which they can be twisted, and these are called S-Twist and Z-Twist. Here is a diagram showing the visible differences. As you can see S-Twist goes upward to the left and Z-Twist goes upward to the right. You can often be able to tell by just looking at the yarn and direction of the plys. Another way of telling is b