As modern multi-tier systems are becoming increasingly large and complex, it becomes more difficult for system analysts to understand the overall behavior of the system, and diagnose performance problems. To assist analysts inspect performance behavior, we introduce SelfTalk, a novel declarative language that allows analysts to query and understand the status of a large scale system. SelfTalk is sufficiently expressive to encode an analyst's high-level hypotheses about system invariants, normal correlations between system metrics, or other a priori derived performance models, such as, "I expect that the throughputs of interconnected system components are linearly correlated". Given a hypothesis, Dena, our runtime support system, instantiates and validates it using actual monitoring data within specific system configurations. We evaluate SelfTalk/Dena by posing several hypotheses about system behavior and querying Dena to validate system behavior in a multi-tier dynamic content server. We find that Dena automatically validates the system performance based on the pre-existing hypotheses and helps to diagnose system misbehavior.