The dictionary says: A person with a high degree of skill in or
knowledge of a certain subject.
Perhaps the question you meant to ask is, how do I tell whether or
not a person is an expert?
Here's my personal answer.
1) I regard someone as an expert if they can show me that they can
accurately convey detailed information on a topic directly from
memory, and when needed can remember or quickly rediscover the source
from which they that got that information. Think of a lawyer who
cites a legal decision, or a minister who can literally "cite chapter
and verse."
2) Someone who has done so much valuable work on a topic that he is
cited as a source by many others, either formally (in a print
citation) or informally (someone telling you "go see so-and-so about
that.")