State coding conflict detection is a fundamental part of synthesis of asynchronous concurrent systems from their specifications as signal transition graphs (STGs), which are a special kind of labelled Petri nets. The paper develops a method for identifying state coding conflicts in STGs that is intended to work within a new synthesis framework based on Petri net unfolding. The latter offers potential advantages due to a partial order representation of highly concurrent behaviour as opposed to the more traditional construction of a state graph, known to suffer from combinatorial explosion. We develop a necessary condition for coding conflicts to exist, by using an approximate state covering approach. Being computationally easy, yet conservative, such a solution may produce fake conflicts. A technique for refining the latter with extra computational cost, is provided.