A fastening system for fastening a child seat or a base for a child seat to a motor vehicle seat, comprises a hook for engaging around a bracket provided on the motor vehicle, such as an Isofix anchorage, in order to couple the fastening system to the motor vehicle seat. The hook is switchable between a first position, in which it does not couple the fastening system to the motor vehicle seat, and a second position, in which it couples the fastening system to the motor vehicle seat. An electrical detection device comprises a sensor suitable for detecting whether the hook is in the first or the second position, wherein the detection device is configured for indirect detection of the respective position or wherein the detection device, in particular the sensor, has a distance relative to the hook, both in its first and in its second position.