Provided is a method for determining at least one characteristic of a boundary layer a wind turbine rotor blade, including capturing at least one movement of at least one flexible element of at least one sensor being attached to or being part of a surface of the rotor blade, determining the at least one characteristic of the boundary layer based on the at least one captured movement of the at least one flexible element. Further, a sensor device, a wind turbine and a device as well as a computer program product and a computer readable medium are suggested for performing the method.