Step 1: Locating faculty
What is your area of interest? Go to the web pages of departments that are related to your interests. Another method is to think about which professors have taught classes that are related to your interests and that you enjoyed. Or ask around for recommendations from other students. Or ask the CHID advisor! Then: Look them up on their departments' web pages. Look for a description of their research interests, classes taught, and publications. You may also want to do a search of the UW website for them. So, for example, when I search on Maria Elena Garcia I find the link that I had seen earlier on the CHID website, but I also discover that she has her own webpage with links to websites to various classes she has taught. If you are lucky, you will find a copy of their curriculum vitae (also known as a "c.v."—the academic version of the résumé). This is good because it will be the most complete listing of the work and interests. A really good idea is to check out their teaching evaluations. You may want to see if students have found them to be fair graders and/or enthusiastic. You want someone that you can work with and who will be fair and invested in your work! This can be someone who knows a lot about your topic and/or with whom you have a great working relationship.
Step 2: Figure out what classes they teach
This is an important step because ideally, you will build a relationship with your thesis advisor before you ever pop the question, "Will you be my thesis advisor?" Check the departmental web pages for this information. Search the course catalog to see if any permanent classes are associated with them. Ask the professor and/or departmental advisor. I would suggest visiting the professor during office hours—they will be able to tell you if they are developing any special topics courses that may be of interest to you as well as their plans for regular courses.
Step 3: What have they written?
A sure way into the heart of any academic is to be familiar with their published work. It's also a good idea to look at what they've written so you can see how they approach their topics. Do they do interdisciplinary work? Can you understand what they have written? This is also a good way to figure out how prominent they are in their field—a useful piece of info in case you are thinking about applying to grad school in their field. So check their home department page for this info—departments frequently include brief biographies of faculty on their websites. Then search the UW looking for their c.v. THEN, go to the library's webpage. Search the library's catalog to see if they have written any books. Search the major databases to see if they have written any articles. Finding articles they have written is even better than finding any books because you can get a better idea of the breadth of their area and see what their current interests are. Books are often their dissertations reworked into publishable form—they can be pretty old by the time they ever get published. You may also want to look at the suggested resources for your area of study.