The receiver of the event can distinguish between the two content modes by inspecting the content-type Header of the Kafka message. If the header is present and its value is prefixed with the CloudEvents media type application/cloudevents, indicating the use of a known event format, the receiver uses structured mode, otherwise it defaults to binary mode.
For Example https://github.com/cloudevents/spec/blob/main/cloudevents/bindings/kafka-protocol-binding.md#331-kafka-content-type