Other than word of mouth, how can you tell who is working in a particular area of research? One way is to look at the professional literature.
The Web of Science allows you to follow citations backward and forward in time, to see how popular particular papers are, who is cited most often, and what subject areas an individual publishes in.
Once you have an idea what you want to study, and who the most important people are in the field, you can identify courses to take, research projects (summer programs?) to work on to prepare you for that field.
Some sites provide a 'best of' list, like SLAC's Top Cites in the area of particle physics.
Here's some news for you. Most of the physics you learn in your undergraduate years is at least 80 years old, and much of it 150-300 years or more. That basic physics underlies much of the current research, but how does it help you to understand what branch of physics you want to pursue?
How can you pick an area of physics that is growing and changing and not one where all the 'interesting physics' has already been studied?
One way is to read the current literature. Several magazines attempt to digest the latest research findings and make it easier for non-specialists to understand. Many of browsable journals are available in the Physics Library, but we also have online access to them.
For those interested in applied topics: