Normally you know what prac it is going to be or the pracs from which they are going to choose. To study for a practical exam it is slightly different from a normal exam.
You want to know things like:
- the method
- variables: independent, dependent, controlled
- how you will summarise your results i.e. a table? do you need to draw any graphs, are there any calculations involved for that practical that you need to be familiar with
- discussion basics: e.g. is the experiment accurate, was it reliable, is it valid. Providing improvements to all those things as well if possible
Those are usually the things tested in a practical exam. If the practical has more theory to do with it, then you may also have questions on that theory as well mixed in some where