The invention relates to a sensor device and a method for investigating a flexible hollow element having an inner surface for interfacing a liquid or a soft medium, the sensor device comprising: - a support (2) to be attached to the flexible hollow element (100); - attachment means (6) for attaching the support (2) to the flexible hollow element (100); - at least one transmitter (3) arranged on the support (2) for exciting acoustic waves in the support (2); - at least one receiver (4, 4a, 4b) arranged on the support (2) for receiving acoustic waves evoked by the transmitter (3), wherein - a signal generated by the receiver (4, 4a, 4b) upon receipt of acoustic waves evoked by the transmitter (3) can be evaluated to verify whether a coating (5) is present between the inner surface (1001) of the flexible hollow element (100) and the medium (7) and/or if a coating (5) is present to determine properties of the coating.