It can be done manually; I've done it from time to time and it's tedious.  You can use Cr2Editor (free), which will show you the obj and textures any character uses, or load them into a Text editor.  Poses can just be deleted wholesale.  Just take care you don't scrap the ones you wanted to keep.  Same with face, hands and cameras.  Oh, and lights, of course.  If a Readme contains only basic instructions for unpacking and a list of files (another place you can find out what goes where), they can also be deleted safely. Same with licenses, although I recommend you keep copies of any Freestuff licenses for reference, just in case you need to know if something is redistributable.

If you have any recent stuff from DAZ, it will most likely have some D|S specific files, which can also be deleted.  You could use Window Search to list them and then delete from there. 

I don't know of a specific app to do what you want but I believe there are several utilities that will show you files and their relationships with each other.

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