We propose a soft palpation sensor to detect anomalies in tissue elastic properties, to enable identification of lesions not visible through endoscopic imaging, such as precancerous colorectal tumours. The sensor includes an inflatable unit which distends the tissue analogous to a palpation task, variations on the sensor surface caused by the tissue stiffness is detected by measuring the electrical impedance within the inflatable chamber.  Palpation experiments were performed where the sensor was attached to a robot arm and compared to an industrial force sensor. The sensor was able to detect 4mm diameter nodules 3mm under the surface, which would not have been visible using visual techniques.