The present invention is a system and method for measuring borehole conditions, in particular for verification of a final diameter of a borehole. The system includes a drill string with a drill bit and a drilling mud circulation device, an underreamer attached to the drill string above the drill bit, and a tool body attached to the drill string, having a sensor for detecting downhole conditions, such as borehole diameter. The tool body is mounted above the underreamer and has a diameter smaller than the underreamer and drill bit. The sensor can be an ultrasonic transducer with a sensor housing for adjustable distance to particular borehole size. The system may also include a calibrator for sensor data, and an auxiliary tool body with another sensor between the drill bit and the underreamer.