OK, looking at some other example code from IBM I think it can be solved by the following.

// Subscribe to the ARTIFACT_OPENED event
RM.Event.subscribe(RM.Event.ARTIFACT_OPENED, function(ref) {
    // Get the artifact attributes
    RM.Data.getAttributes(ref, RM.Data.Attributes.FORMAT, function(opResult) {
        // Check previous call completed OK
        if (opResult.code === RM.OperationResult.OPERATION_OK) {
            // Load an array with the attributes of the artifact
            var attrs = opResult.data[0];
            // If the artifact is a module (the value of the attribute 'format' of the artifact is identical to that of a module)
            if (attrs.values[RM.Data.Attributes.FORMAT] === RM.Data.Formats.MODULE) {
                // call a function to do something with the module having handle 'ref'
                processModule(ref);
            } else {
                // it's not a module so do something else (or do  nothing) with the artifact
                doSomethingElse(ref);
            }
        }
    });
});