In the concurrent distributed language Erlang, processes interact through message passing and signals. One of the main sources of non-determinism that make Erlang programs difficult to debug, is the order in which messages are handled. We present a prototype tool called CESAn that implements a small-step semantics that faithfully represents message handling in Core Erlang, enabling the user to investigate the causes of non-determinism in Erlang stemming from message handling. We see CESAn as a building block for debugging and analysis tools for Erlang .