In this paper we deal with the observability problem in discrete event systems modeled with interpreted Petri nets (IPN) under partial state observations. In particular, under the assumption of event-detectability and the knowledge of the number of system resources, we provide a methodology to compute the estimations reachability graph for a kind of IPN. Later, the estimations reachability graph is used to analyze some observability issues of a net such as the observability constant, the convergency ring sequences and testing a net for observability.