What is a common way to tell if an academic is "above average," or outstanding in their field? Is their h-index (Hirsh index) one of them? If one claimed that a particular scholar was "above average" or "noted" in their field, is there any good metric by which to support or deny such a claim?

Is there a generally accepted way to indicate that a particular professor or scholar is outstanding, or above average, in their field? I understand there are certain indicators, such as chairs, endowments, prizes, etc. But these don't really seem to help to compare one scholar to another, except in a sort of gross, simple count way (i.e. one professor has had more chairs than another)

Is it theoretically possible to create a "ranking" of professors in their fields, by some metric? Could their h-index serve as such a metric?
Yes, there is one and only one standard method that is universally employed by reputable academic institutions worldwide.  This is how you evaluate a researcher:


Read their papers.
Attend one of their talks.
Ask the opinion of other experts in the field.


This is how hiring committees and promotion committees do their job.  There are no shortcuts.  Parts 1-2 require that you have some relevant expertise; if not then you must rely entirely on #3.  

Every academic is regularly asked to give an expert opinion through reference letters, which often are expected to include some sort of ranking (e.g. "Assistant Prof. X should be promoted because she is clearly as talented/better than Prof. Y who was recently promoted at Prestigious University Z").  How does one justify this kind of claim in the reference letter?  You guessed it:


Read their papers.
Attend their talks.