Serious games are increasingly used in schools, universities or in vocational training. When they are used in the classroom, teachers often have to deal with the lack of tools for monitoring the students during the game and assessing them after the game. So they often tend to add assessment questionnaires to the fun sequence of " learning by playing " , to ensure that students have learned during the session. Our goal is to enable the teacher to do without this type of questionnaires by providing them an automated tool for monitoring and analyzing the actions performed by learners. The system combines an " expert Petri Net " and a domain and game action ontology. Our first experiment conducted on a sample of fifteen students showed that the diagnostic tool gives relatively close results to those of an online assessment questionnaire proposed by the teacher.