In open systems, the verification of properties of subjects is crucial for authorization purposes. Very often access to resources is based on policies that express (possibly complex) requirements in terms of what are referred to variously as identity properties, attributes, or characteristics of the subject. In this paper we provide an approach that an entity called a verifier can use to evaluate queries about properties of a subject requesting resources that are relevent deciding whether the requested action is authorized. Specifically, we contribute techniques that enable reuse of previously computed query results. We consider issues related to temporal validity as well as issues related to confidentiality when one entity reuses query results computed by another entity. We employ constraint logic programming as the foundation of our policy rules and query evaluation. This provides a very general, flexible basis, and enable our work to be applied more or less directly to several existing policy frameworks. The process of evaluation of a query against a subject identity is traced through a structure, referred to as identity proof tree, that carries all information proving that a policy requirement is met.