playbin3, decodebin3: how to get information about unsupported streams

We are using playbin3 for a mediaplayer implementation. We play a multiplex with audio and video and for video there is no decoder available. Then playbin3 creates a valid pipeline with only audio sink. So far so good.
Is there a recommended way how to get information, that not all streams from demux element are decoded? The "old" decodebin was emitting an "unknown-type" signal in that case. This is no longer available in playbin3

Thanks for any hint
/Juergen