Add a Usage (as named in DaVinci Resolve, ? in Kdenlive) to the LOV for Columns in the Playlist menu.
To the uninitiated the Usage column shows how many times a video or audio is on the timeline to assure the editor that the clip in Source has been added to the timeline when the telephone rings or any other distraction happens.
This is what Blackmagic Design calls this. KDE have this as simply the number in brackets.
The next version adds bins and will have a “Not In Timeline” smart bin. I can add an “Already In Timeline” smart bin after that version (beta is ending soon). Cannot say whether there will ever be a usage count as I think that can have prohibitive performance overhead. Every time the timeline changes it will need to be re-inventoried and check if each playlist item’s count changed. I have heard of people using thousands of playlist items and tens of thousands of timeline clips. I do not know how the others are doing it, but I know that nearly everyone complains about the performance of their video editing software. I choose to be cautious and pragmatic in that regard rather than try to keep up with the Jones’.