Good question.  Whether by regulation or professionalism it is important to be sure that you have acted in a skilled and prudent manner. To me part of diligence would be never assuming a client knows how tax regulation relates to their business at a deep level. Be sure that your own experience is sufficient for a clients needs. For example if you a client with extensive real estate holdings be sure you understand the nuances of real estate well. Pre and post in service, fair rent/personal use requirement, land/building allocation, exchange and installment handling and elections, and so on. With that understanding you can ask a client specific questions and quickly determine whether the numbers you are working with are not only accurate but appropriate for the code requirements of the return you are preparing. Obviously you are not going to do an audit of every clients books but with some well placed questions and data inspection you can determine if the clients knowledge and ethics are a good match for your own strengths and skill set.