If you're a litigator who works with expert witnesses, you'll learn pretty quickly that experts who look similar on paper can be very different in person. Even controlling for exaggeration of credentials and experience, differences between two seemingly evenly matched experts can have a profound effect on their ability to help you prepare a case and persuade a jury.
What should you look for that can help you distinguish a stronger witness from one who seems comparably qualified? While many factors can potentially affect the expert's performance, here's a quick and dirty list of some important ones:
Whenever possible, your expert's degree should be relevant to the subject matter of the testimony. Your expert should have meaningful credentials, not credentials that seem impressive when presented as a list but which were obtained through a minimum of effort, perhaps even by simply paying a fee. The more difficult it is to obtain and keep a credential, the more likely it is that your expert is going to truly hold the skill set implied by that credential.
Does your expert have certifications that can only be obtained through experience and testing, or that have a continuing education requirement?
Are the expert's professional memberships for organizations that anybody can join, or do the organizations have additional requirements relating to expertise, experience, and qualification?
Are members tested, or required to engage in regular training?
Ask your prospective experts for some examples of reports he has made. Ideally the reports will be from other cases, but to the extent that your expert cannot provide those reports due to confidentiality agreements or due to limited experience as an expert, the expert should be able to provide reports he has prepared in other contexts.
Are the reports well-written, well-organized and well-illustrated?
Are the reports convincing?
If the reports reflect considerable thought and quality, should you require a report, you can be more confident that the expert will prepare a report of similar quality for your case. If they reflect a formulaic approach, are not well-written, are not persuasive, or are not well-organized, even if you don't intend to ask the expert for a report you'll gain insight into the quality of your expert's work product. An expert who provides low-quality examples of his work is unlikely to perform any better when asked to help you develop your case, when the expert prepares reports for you or testifies.
Does the expert charge reasonable fees, consistent with other experts in the field? Are provisions for reimbursement of expenses fair or excessive? Does the fee agreement include hidden or unexplained costs or charges?
Although it's not always indicative of a problem, when your expert only works for one side in litigation -- always working for defendants, or always working for plaintiffs -- the pattern may suggest problems to come. Beyond the obvious question, will a jury view an expert who always represents one side as a credible and objective witness, you need to consider the possibility that the expert has formed biases based upon that one-sided experience. It's possible that the expert has preexisting biases that are reflected in the expert's consistent decision to turn down any work offered by the other side.
As you know, a big part of trial work is showmanship. If you have two evenly qualified experts, you need to consider how well each of them will present to the jury. It may seem shallow to focus on appearance, dress, vocal tone, posture, eye contact, mannerisms, and the like, but they affect how a jury perceives an expert.
Also important, and far less superficial, are questions of how well your expert listens and answers questions. A good expert is careful to recognize what is being asked, and is capable of providing a clear and concise answer to a question, breaking down jargon into understandable terms and not answering beyond the scope of the question. An expert who lacks those skills is more likely to become confused or flustered in court, to provide answers that are difficult to understand, and to testify beyond the scope of a question, potentially volunteering information to the opposing party that they would not otherwise have obtained.