I don't think there are any all around fishing experts. I think you have to qualify the word expert with the area or areas that person specializes in.

For example, a guy may be one or more of the following:
drift fishing expert
boon dogging expert
plug pulling expert
spey casting expert
Lake WA cutthroat fishing expert
Spoon fishing expert
nymph fishing expert
Jig\bobber fishing expert
Green river steelhead fishing expert
etc...

The more of these types of things a guy has under his belt, the closer he comes to being an all around fishing expert - but I've yet to meet someone quite that good...

It's like asking if someone is a 'medical expert'. More than likely, they will have a lot of general knowlenge, and or they will have a specialty that they may be considered an expert at (foot doctor, eye doctor, ...)

So then the next question would be: How do you tell if someone is an expert at a specific type of fishing? Success rates? Flexibility\adaptabilty? Years experience? Out fishes the 'competition'? Better at it than you? Good form, technique, or presentation? Good teacher? All or some of the above?