Slaty foliation, is a type of parallel foliation consisting of fine-grained platy minerals. The direction of foliation is usually perpendicular to the direction of maximum stress.

Foliation is the arrangement of minerals in parallel layers.

The foliation in metamorphic minerals is always perpendicular to the direction of pressure. E.g. Vertical pressure is applied, the foliation will be horizontal, and vice-versa, the pressure is horizontal you get vertical foliation. From yahoo answers

Slaty foliation, is a type of parallel foliation consisting of fine-grained platy minerals. The direction of foliation is usually perpendicular to the direction of maximum stress.

I am doing a lab and was wondering the same thing. I found out that you have to look at the rocks foliage. If it is foliated or nonfoliated. ( foliation- a process which minerals are aligned in rocks such that they are easy to split either in half or small thin slices).