A test cavity
The test cavity method for assessing pulp vitality is not routinely used since, by definition, it is an invasive irreversible test. This method is used only when all other test methods are deemed impossible or the results of the other tests are inconclusive.
An example of a situation in which this method can be used is when the tooth suspected of having pulpal disease has a full coverage crown. If no sound tooth structure is available to use a bridging technique with the electric pulp tester and  cold  test  results  are  inconclusive,  a  small  class  I  cavity preparation is made through the occlusal surface of the crown. This is accomplished with a high-speed #1 or #2 round bur with proper air and water coolant. The patient is not anesthetized  while  this  procedure  is  performed,  and  the  patient  is asked  to  respond  if  any  painful  sensation  is  felt  during  the drilling procedure. If the patient feels pain once the bur contacts sound dentin, the procedure is terminated and the class I cavity preparation is restored. This sensation signifies only that there is some viable nerve tissue remaining in the pulp, not that the pulp is totally healthy. If the patient fails to feel any sensation when the bur reaches the dentin, this is a good indication that the pulp is necrotic and root canal therapy is indicated.
Cohen's Pathway of the Pulp ed:11th Pg20
is the  first test in diagnostic sequence.
often results  in a dull   pain response.
is employed only when all other test findings are equivocal.
should be performed  with local anesthetic.