What qualifies an expert witness? How is an expert qualified and who qualifies him?

If the person went to school and got a degree for what they are testifying to.
Like a forensic scientist is an expert witness that the DNA matches.
A psychologist is an expert witness that a person displays multiple personalities.
A fire marshal is an expert witness that a fire started because of faulty wiring
An insurance adjuster can claim that a problem existed or happened as a result of...

Their experience and training, for example, I am trained by course work approved by the POST ( Police training) in accident investigation, and have investigated as a police officer many accidents. So I could be hired as a expert witness ( and have been) to testify about accident reconstruction, and possible at fault methods.

If you are a medical doctor licensed with the AMA you would be considered an expert allowed to testify, That does not mean you are a good witness, unless you are experienced at testifying in court.

A fingerprint expert will have training accepted by the court as a finger print witness.

What qualifies an expert witness? How is an expert qualified and who qualifies him?
As I recall, the Supreme Court has said that anyone who has "above average" knowledge of a subject can be considered an "expert" and the validity of the testimony rests with the jury and the judge is ultimately responsible for determining who is considered an "expert"

Therefore, almost everyone is an expert at *something*