This essay tries o develop classification criteria to identify smoke-free spaces using environmental monitoring, direct inspection and worker reports, comparing their agreement and deriving a proposal useful for the epidemiological surveillance of environmental tobacco smoke. Environmental nicotine monitoring, direct inspections and workers surveys regarding tobacco smoke presence were conducted in ten institutions. For each method, criteria were defined to classify institutions as smoke-free spaces. Results were compared to evaluate between-methods agreement. Good agreement between environmental monitoring and direct inspections were observed, although they disagreed in 20% of the cases. Worker reports were too frequent to discriminate. Combining environmental monitoring and inspection would provide the most sensitive classification. Cost-effectiveness studies are required to identify the best strategy.