Biblatex uses a 'crossref' field to link entries together. For example, a chapter in an edited volume will be of type 'incollection' and have a crossref to an entry of type 'collection'. When cited the details of both are used in the bibliography. This script looks at a biblatex file and checks that everything with a crossref field has a corresponding entry, and is of a suitable type. You will need to have installed the Python bibtexparser package. The --expand option modifies the file by copying the information from the referenced entries into those that reference them. A backup is made first.
This script takes a markdown file and biblatex file as arguments. It checks that every reference in the file occurs in the bibliography. If the --unused option is given then instead the script checks whether there are items in the bibliography that aren't used in the markdown file.
The functionality here has been replaced by github.com/twsh/Convertbibliography.