There are no options to search for blocks instantiated in this way, if a block is unconnected it will probably still have components and connectivity inside the block so this is not easy to track. What is the use case for such a check?
Thanks Robert for your answer.
About the use case you asked for: if I'm right, the user had inherited a design from another designer and would like to check whether all blocks are completed or used.
I will report this back to the specific user.