Qualitative fit testing is a pass/fail test method that uses your sense of taste or smell, or your reaction to an irritant in order to detect leakage into the respirator facepiece.
Whether the respirator passes or fails the test is based simply on you detecting leakage of the test substance into your facepiece.
Qualitative fit testing is normally used for half-mask respirators - those that just cover your mouth and nose. Half-mask respirators can be filtering facepiece respirators - often called "N95s" - as well as elastomeric respirators.