An activity tracking system is configured to receive an activity report identifying a fitness-related action that is reported to have been completed by a participant. Upon receipt of the activity report, the activity reporting system verifies the workout, e.g., by obtaining data that can be used by the activity reporting system to determine whether the activity report represents actual activity as reported by the participant or, in the alternative, suspect activity. Based on the data obtained from the one or more data sources and the information in the activity report, the activity tracking system determines a probability that the activity report represents legitimate fitness activity. This probability determination may then be output or reported to other systems (e.g., fitness history databases, websites, and the like, third party systems (e.g., incentive systems, reward systems, and the like).