UCR fees (usual, customary and reasonable) are terms used by insurance
companies to determine limits of coverage.  It is defined as the 90th
percentile for a given area, but insurance companies are notorious for
using UCR tables that are woefully out of date.
    As far as guided tissue regeneration (GTR), this is a procedure done
primarily by periodontists, so I'm not familiar with typical fees.  I
would imagine it would vary widely, depending upon how many teeth are
involved, whether bony grafts are used, etc.  As such, while I'm sure
insurance companies have a figure (they tend to keep these figures a
deep dark secret, btw), they really aren't useful to patients, I'm
afraid--there are really just too many variables to the procedure.

Steve

Can you tell me what the usual and customary fee for Guided Tissue
Regeneration is?

    I wasn't trying to be facetious.  There isn't any practical way for you
to evaluate dentistry on your own.  You will know if it hurts, you will
know if something falls out.
    Even another dentist will not be able to tell after the fact if
restorative treatment (no matter how nicely done) was necessary, unless
(and sometimes not even if) pre-operative x-rays are provided.

Steve

Consulting another dentist is easier said than done.

Also, how does one know if the work the dentist said they performed was
really done?

    Consult another dentist.

Steve

How does one know if the treatment recommended by their dentist is really
necessary.  Also, how would one know whether the work they said they did was
really done?