The dentist jams a scaling tool into the cavity very hard and notices the
patients response.
That's the way a military dentist did with me.
After a few of my words like, "I told you that the tooth hurt, you son of a
b---- mother f------", he extracted the molar.
Jamffer

    The nomenclature is a bit confused, to be sure.  When we were in school
in medieval days, we were told that the pulp was incapable of recovering
from significant inflammatory changes.  The reaction to drilling that
was considered reversible was called "transient hyperemia" which is a
purely empirical description.  In a transient hyperemia which is most
frequently seen after treating a cavity, the primary symptom is
exaggerated response to hot and especially cold.  There may be pain,
even severe, but it vanishes within a few seconds.  If it's throbbing
for ten or fifteen minutes after this we infer that the nerve is in
trouble.
    But these designations aren't black and white, and I've seen situations
with symptoms somewhere between these two that eventually calm down.
Now, a certain percentage of these will later show that the nerve has in
fact degenerated, but some of these teeth remain assymptomatic (and we
assume have vital and healthy pulps) indefinitely.

Steve