Verification techniques are applied to policy based systems to ensure design correctness and to aid in the discovery of errors at an early stage of the development life cycle. A primary goal of policy verification is to evaluate the policy's validity. Other analyses on policy based systems include the identification of conflicting policies and policy efficiency evaluation and improvement. In this work, we present a discussion and classification of recent research on verification techniques for policy based systems. We analyse several techniques and identify popular supporting verification tools. An evaluation of the benefits and drawbacks of the existing policy analyses is made. Some of the common identified problems were the significant need of computational power, the limitation of the techniques to particular policy model, which restrict their extension to other policy models and the lack of efficient conflicts resolution methods. We use the evaluation results for discussing the further challenges and future research directions that will be faced by policy verification techniques. In particular, we discuss specific requirements concerning verification techniques for coalition policies systems and autonomous decision making.