A tub drain overflow test system employs a kit with a connecting pipe with a front cylindrical portion having an inner threading and an intermediate annular flange coaxial with the front cylindrical portion. The front cylindrical portion extends through an overflow hole of a tub so that the annular flange, optionally with a seal ring, abuts the rear surface of the tub wall. A test cap has outer threading that mates with inner threading of the front cylindrical portion. The test cap has an annular flange and a breakaway test seal with a projecting grip tab. The test cap has an inner lip that provides a catch for inwardly extending tabs on a fixture to attach. The test cap allows the overflow drain to be tested. A seal portion is removed to provide access to the opening and the finishing fixture is mounted in place.