I cannot tell you that the tooth needs root canal or not.  However,
these symptoms are totally consistant with an abscess and a tooth
requiring root canal.
    There has been a definite movement toward doing root canal treatments
in fewer visits.  Most root canals can now be done in one visit.  Part
of this is the new rotary instrumentation, which makes cleaning of the
canals quicker and easier.  Part of it was a mountain of evidence that
multiple visits (and culturing of canals which used to be done
routinely) did NOT lead to better results.

Steve

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?