The validity of sensor data of an Ethernet onboard network in a motor vehicle is checked by: determining a delay time of a first signal on a first connecting path between a first control unit of the Ethernet onboard network and a second control unit of the Ethernet onboard network; determining a maximum speed of the first connecting path on the basis of the delay time; and determining a type of a transmission medium of the first connecting path on the basis of the maximum speed. The following steps are also carried out: identifying at least a first control unit of the Ethernet onboard network, synchronizing at least a first control unit of the Ethernet onboard network, ascertaining the synchronization interval, ascertaining a drift of a timer of the first control unit, ascertaining a timestamp of the first control unit, reading a timestamp of the first control unit.