Poorly.
The relevant legal rule is
here in the Federal Rules of Evidence. The controlling legal precedent for interpreting the rule is the
Daubert standard. In practice, neither judges nor attorneys are good at determining whether a witness is a true expert and whether expert testimony is based on a solid foundation of expertise, fact, and reliable method. And they know this about themselves, which means they tend to err on the side of accepting expert opinion as evidence. Loss Leader recommended me the book
The Cadaver King and the Country Dentist, which illustrates just how far one can go as an "expert witness" in the American justice system.
In practical terms, a potential expert witness for either party would submit a c.v. to the court and be qualified pretty much on those grounds alone as able to give expert testimony.
Voir dire of proposed experts is allowed. It would be up to opposing counsel, either in pretrial hearings or at trial, to poke holes in either the foundation of the expert testimony, or the testimony itself. But there's nothing really that the judge would do to verify that experts really are experts. Most often, the plaintiffs present their experts and the defendants present their own experts, and the jury decides whose opinion is the most credible.
