Collaborative systems are growing in use and in popularity. The need to boost the methods concerned by the interoperability is growing as well; making thus trustworthy interactions of the different systems a priority. The systems need to interact with users and with other applications in trusting each other. The decision regarding with who and how to interact with other users or applications depends on each application or system. In this paper, we focus on "soft trust", that is trust management systems based on observations of the trustee behaviors to evaluate the trustee experience. Furthermore, we propose a formal distributed network monitoring approach to analyze the packets that the trustor and trustee exchange in order to prove the trustee is acting in a trustworthy manner. Based on formal "trust properties" defining the analyzed systems, the monitored systems behaviors on which these properties are checked provide, through testing verdicts, an evaluation of the trustor/trustee. Finally, our methodology is applied to a real industrial DNS use case scenario