It is difficult to comment on this case without further info, like
diagnostic tests done before root canal was performed on that tooth,
to ensure that the tooth was the culprit, as well as x-rays of both
teeth taken before and after the root canal.  Maybe both teeth were
infected, but only one showed symptoms at the time of the root canal.
Maybe the tooth worked on was the wrong tooth, and the tooth behind
it was the culprit.  Usually a dentist should cut down the tooth
having the root canal so that the patient doesn't bite into it, which
would cause sensitivity after a root canal.  Or maybe the dentist
extruded debris from the inside of the canal to the outside of the root
inside your jawbone during the root canal, which can lead to temporary
sensitivity after the root canal.  Usually the dentist should have
detected the "shadow line" around the other tooth if his x-rays taken
during the root canal
showed both teeth.  Maybe the sensitivity on the back tooth is due to a
residual infection on the tooth that had the root canal.  But basically
if the
pain on the root canal tooth goes away, things will be okay, as long as
all the canals in the tooth were cleaned and there were no perforations
in other roots in the molar tooth.

Suprisingly, dentists don't seem to know why exactly some patients
feel a lot of sensitivity temporarily after a root canal, even if the
root canal was done with textbook perfect results.  Sensitivity after a
root canal done properly can be due to the patient biting down on the
tooth after the root canal, or can be due to debris being pushed out
the
end of the root during the root canal (a common event).  It could also
be due to the root canal filling forcing the tooth infection outide the
tooth, where it might temporarily swell up and make pressure at the
root end of the tooth, causing sensitivity.  the sensitivity would go
away once the body's immune system came in and destroyed the remaining
infection.

it can be a good root canal and still give this kind of pain.  but
also the dentist might have worked on the wrong tooth or accidentally
failed to clean out one of the canals in the molar.  can't tell in this
case without full info.

Not really.  Some dentists market themselves well, others are liars.
some are competent but poorly marketed.  others are competent but not
meticulous enough to do a good job.  others are burned out.  dentists
who are university instructors may be slightly better than average but
this may be hit or miss. This profession is not as scientific as it
should be.  It is influenced mainly by gurus who usually are right but
not always, who often don't admit it when they are wrong.  experience
is a plus, at least 5 years would be nice.  but one is not too young to
be competent or too old to be incompetent.

However, it is fairly easy to evaluate root canal cases to see if
they are properly or improperly done, just bring a record of x-rays
taken before and after root canals, as well as record of diagnostic
tests, to another dentist, especially a specialist root canal doctor
(endodontist) and you will probably get a clear answer about if it was
done right or not.

About two months ago I had a root canal on the next to last back molar. This
was done by my dentist in one visit.

For several days after that I had what felt like extreme soreness on that
tooth whenever I applied any pressure. However that finally went away.

Surprisingly I had the same soreness/sensitivity at the last molar right
next to it which continues intermittently through today. If I press the gum
with my finger below that tooth it feels very sore to the touch and
generates much more discomfort than simply applying pressure to the tooth
itself. Pressing the inside of the gum generates no soreness.

Have gone back to the dentist and comparing x-rays now of that tooth to one
taken just prior to the root canal you can detect a shadow line around the
bottom of the root. He believes that I now need a root canal on that tooth.

The only reason I have doubts is because I have never experienced this type
of tooth pain before and wanted to be sure before having yet another root
canal. I also have a question about root canals in general. I have had two
in my life the first one nearly 20 years ago. At that time it was done in
three visits and I recall no further pain after the initial or the follow up
visits so I was surprised how much pain I had after the second which was
quite unpleasant. Is that common and does it have something to do with the
method used today?

So the question, is this type of pain consistent with the root going bad? If
not what should I be asking the dentist or should I be seeking a second
opinion?

On that latter note short of word of mouth is there anyway to determine who
is a good dentist?