From Webster's Third New International Dictionary:
expert - "one who has acquired special skill in or knowledge of a particular subject through professional training or practical experience"

Personally, I'd define an expert as "someone who can skillfully solve problems and get desired results in a speciallized area of operation, regardless of training or experience"

> are all experts self-appointed?

Don't know about that, but I would say that any true expert knows that he or she is an expert.

> is there some criteria for calling yourself one?

Per the definitions above, you'll know you're an expert when (1) you have acquired specialized skill and/or knowledge, and (2) you can use your skill and knowldege to solve problems and get desired results in that specialized area of operation.

>how do you know you are one??

Same answer as the last question. PERSONAL EXAMPLE: I have done sheet metal work for about 11 years. There came a point a few years back where I realized that there is no sheet metal problem I can't solve. That makes me a "sheet metal expert". Does it mean that I know everything there is to know about sheet metal? Not by a long shot! It simply means I have acquired enough knowledge and skill to be able to solve any sheet metal problem I come up against. In short, I can get desired results in sheet metal --- every time! However, it is of interest to note that in attacking sheet metal problems, I sometimes have to consult with other sheet metal experts to get the information I need to solve the problem. This is part and parcel of being an expert: Knowing your limits (knowing what you know and what you don't know), and where to go to get answers that are beyond your current realm of knowledge, skill or experience. That gives you, as an expert, the opportunity to expand your expertise! Pity (and avoid) the "expert" who thinks he or she knows everything there is to know about a subject.

--- Hugh