I think you'll get a lot of good feedback here, so I take a different lens and focus on seller him- or herself. You can't compartmentalize dishonesty, deceit, or fraud. If you find this with your seller in one area, it likely also exists elsewhere. It also gives you good incite into the seller's ethics and integrity. As one of my investors says: "If someone is willing to lie and cheat the federal government--a body that can do terrible things to a person--on something like taxes, how will that person treat you? How honest will they be with you?"

In no order/priority: strength of management (particularly those that will stay), customer concentration, related party transactions, confidential survey of key customers, defensible assumptions for their 3 year go forward plan, how often they competitively quote all vendors, QE, environmental issues, customer history (years), core competencies (vis a vis competitors), legal and regulatory issues, remaining life of IP, what are the owner's KPI (their dashboard), trends in gpm% and why its changes, for starters. Agree with Joseph: you can hire investigative services to trace, profile the key players.