Coverage analysis of programs and specifications is a common approach to measure the quality and the adequacy of a test suite. This paper presents a probe insertion technique for measuring the structural coverage of LOTOS specifications against validation test suites. Coverage results can help detecting incomplete test suites, a discrepancy between a specification and its tests, and unreachable parts of a given specification. Such results are provided for several examples, taken from real-life and hypothetical communicating systems for which a LOTOS specification was constructed and validated.