Was referred to a client by a person I trust.  He needs business tax returns (2017-2019) filed for a laboratory that never made sales. I know that a different CPA reviewed a draft tax return that the client prepared but preferred to hand it off as he doesn't do businesses.  He admitted that he has filed a lawsuit against his equipment supplier.  His CPA and accounting manager for the lab both died a few years ago and he doesn't have good financial records. So those facts give me pause to determine what due diligence is needed.  Other than a discussion with the potential client, a review of the accuracy and completeness of the accounting records, what sort of investigation can I do?