Apparatus for use in checking the fit of a respirator with the face of a wearer. The apparatus may include a generally rectangular envelope having a longitudinal opening to receive the respirator therein. Low adhesive contact surfaces may be formed within the interior of the opening to form a fluid barrier with the periphery of the respirator. One or more passageways, formed within the apparatus, may be opened and closed to test the fit or seal between the periphery of the respirator and the face of the wearer. The envelope may be used to check the fit of a wide variety of types and styles of respirators and face masks. Envelopes with configurations other than rectangular may also be satisfactorily used to check the fit of a respirator.