Look through the job listings, and learn how jobs in your discipline are defined. In political science qualifying exams cover three sub-fields. The choice of your first and second qualifying exams is usually very easy to make based on your course work and interests. The choice of the third is often more of a flip of a coin between two or more possibilities. Seeing how jobs are defined can help you make that choice in a way that has a positive impact on your later job search.
Fads come and go.
And the idea is not to structure your graduate education around your perceptions of where the job market might be when you finish, rather it is to tinker about the margins in a way that will benefit your professional development when you become an ex-graduate student. Again using political science as an example, there are many times more annual openings in American politics than political theory. The ability to teach in areas of high demand or low supply can help separate you, especially in small departments, from other viable candidates. In a similar vein, begin to attend workshops offered by your department, grad division and the career center. Since you are so far removed from being on the market, you can listen and evaluate without the immediate pressure and panic that comes from learning about the process for the first time when it counts for real.