Testing ConsumerSupervisor In GenStage pipeline

I have a GenStage pipeline that looks like the following

       A  (producer)
       B (producer-consumer)
       C (ConsumerSupervisor)
    /  |  \

I would like a way to test that C (my ConsumerSupervisor) is in fact starting a consumer for each event emitted from stage B.
Any suggestions on how to test this part of my GenStage pipeline in isolation?