a method and device for detecting an error rate of a receiver of data transmitted to a device that is to be checked is provided . a test device generates a data block having a first length and a test signal generated based on the data block is sent . the device to be checked receives and evaluates the test signal . another data block having a different length is generated based on the evaluated test signal to obtain another data rate . the other data block is sent as a response signal in a different transmission direction , and is received and evaluated by the transmitter / receiver device of the test device . an evaluation unit compares the contents of both data blocks to detect the error rate , wherein the content of the shorter data block is compared with the content of a corresponding segment of the longer data block to determine the error rate .