When the Word Leads Your Pastoral Search: Biblical Principles and Practices to Guide Your Search suggests a number of areas to probe in the interview. You should interview candidates about plagiarism in the pulpit. A series of posts on the Gospel Coalition web site will help you better understand the subject.
When I was doing my doctoral research on the subject of churches calling a pastor, I asked a district leader to suggest interview questions. He suggested was that candidates should be asked:
There is a range of opinions about what is acceptable as an answer to this question. The matter is complicated because some well known pastors encourage others to use their materials without giving credit. When I was doing research on this book, one district leader told me of a pastor who lost his job because he was using someone else’s materials and disagreed with his church leadership about what constitutes plagiarism.
Those looking to better understand this subject can look to a series of posts on the Gospel Coalition web site. Collin Hansen gives a very helpful outline of the problem (see here). He then asks a number of well known preachers to define what constitutes plagiarism in the pulpit.