Any advice for evaluating the outstanding liabilities a target may have incurred by not having its IP fully cleared?

At the end of the day, this is a simple expected value calculation: Probability of being successfully sued for infringement x The amount of the damages. Unfortunately, there is no database of successful versus unsuccessful infringement actions so it's impossible to quantitatively outline this measure. Typically you have to rely on a qualitative understanding of the type of IP, the quality of IP "chain-of-custody", an assessment of any external parties involved in the IP's creation, and the company's appetite and ability to defend itself in an action. Things to look for include IP assignment agreements with all employees and contractors involved in creating it, a review of all of the EULA's and license agreements, a detailed history of how the IP came to be and a view of past actions that have occurred and how they typically get resolved in the legal process. Combining all of these factors will help drive a rough view of potential liabilities. Of course, one way to help mitigate this risk is to have the target provide indemnification for any IP liabilities, but understand that their ability to indemnify is limited to the resources they have to address a potential action and finding. So while it provides a mechanism for insulation, it may not fully protect if the liabilities are significantly large enough.