Biographies that you want displayed on your website, kennel cards, and sent to adoption aggregators like Petfinder and Adopt A Pet will be entered into a Kennel Card / Website memo on the animal record. In order to get a report of the animals that do not currently have a biography, you'll want to run a Custom Entities Animals in Custody repot, adding the Kennel Card / Website Memo to be included in the results. The blank rows in the memo column will identify which animals still need a biography.
You can also export this to excel if you prefer to filter the results to just display the blank rows for the Kennel Card / Website memo.