In this paper, we present an approach that helps in the design and assessment of detectors. A detector is a program component that asserts the validity of a predicate in a given program state. We first develop a theory of error detection, and identify two main properties of detectors, namely completeness and accuracy. Given the complexity of designing efficient detectors, we introduce two metrics, namely completeness (C) and inaccuracy (I), that capture the operational effectiveness of detector operations, and each metric captures one efficiency aspect of the detector. Subsequently, we present an approach for experimentally evaluating these metrics, and is based on fault-injection. The metrics developed in our approach also allow a system designer to perform a cost-benefit analysis for resource allocation when designing effecient detectors for fault-tolerant systems. The applicability of our approach is suited for the design of reliable component-based systems.