Using git fsck to display dangling and unreachable objects.

git fsck [--tags] [--root] [--unreachable] [--cache] [--no-reflogs]
         [--[no-]full] [--strict] [--verbose] [--lost-found]
         [--[no-]dangling] [--[no-]progress] [--connectivity-only]
         [--[no-]name-objects] [<object>*]

In part:

<object>
    An object to treat as the head of an unreachability trace.

    If no objects are given, git fsck defaults to using the
    index file, all SHA-1 references in refs namespace, and all
    reflogs (unless --no-reflogs is given) as heads.

--unreachable
    Print out objects that exist but that aren’t reachable from
    any of the reference nodes.

--[no-]dangling
    Print objects that exist but that are never directly used
    (default).  --no-dangling can be used to omit this
    information from the output.

--unreachable overrides --no-dangling.