Literature often relies on ex post feedbacks from participants to investigate how public participation affects social capital. However, some scholars suggest that the actual achievement of own interests might bias the participants' ex post perception. Therefore, we designed an on‐going evaluative system to monitor the dynamics of trust during public participation, and we tested it in a real case of engagement with local community of stakeholders. The evaluative system could track both processes of creation and destruction of trust over the time, designing complex patterns of evolutions. This study also provided hints for a more effective use of participatory tools.