Tactile sensing is paramount for robots operating in human-centered environments to help understand interactions with objects. One challenging issue in tactile sensing is the slip detection problem. Currently, most slip detection methods use passive sensors, which only measure force or vibration, but not directly the contact area, which is one of the most reliable parameters for slip detection. Other methods, which do measure contact area such as optical methods often suffer from complex sensor configurations and lack of flexibility for customization. Active sensing, in contrast, has simple sensor configurations, and can also directly measure the contact area to improve the efficiency for slip detection. In this article, a novel active sensing smart skin was developed for incipient slip detection, which leverages piezoelectric transducers as actuators/sensors. First, a robotic fingertip with embedded actuator and sensor was created in which the actuator generates ultrasonic guided waves received by the sensor during a slip scenario. By analyzing the received signals using an attenuation-based method, the entire contact condition evolution can be monitored within 1 ms. Then, an optical method was used to validate the signal attenuation, which correlated consistently with the contact area. Finally, a unique robotic skin was created which demonstrated robust and sensitive response for incipient slip detection.