LawyersUSA.com reporter Justin Rebello wrote an article on finding expert witnesses. In the article he interviewed Michael Brennan a research analyst at law firm who specializes in researching expert witnesses. He keeps a computer database containing 7,500 categories of experts.
He discusses going to Amazon or the Library of Congress and search for book on it. There's a very good chance the person who wrote the book is an expert. Colleges and universities are a great resource if you need experts in physics or whatever it might be academically.
For researching opposing witnesses - it's mostly fact gathering for attorneys to see what their background and expertise is.
Three things cited for researching expert's educational background, their professional licensing and their association memberships.
To determine if experts will make for good witnesses - there is a recommendation to look for teachers, such as a college professor, who have experience explaining a concept to an audience. Consultants also have the skills to communicate to an audience.