Assignment outsourcing, also known as contract cheating, occurs when a student outsources an assessment task or a part of it to a third party. It has been one of the most pressing ethical issues in university education and was further exacerbated by the wide availability of chatbots based on large language models. We propose a method that has the potential to verify the authorship of a document in question by filling in a cloze test. A close test with 10 items selected by our method can be used as a classifier with an accuracy of 0.988 and a F₁ score of 0.937. We also describe a general method for building a cloze-test-based classifier when the probability of authors and non-authors correctly filling in cloze items is known.