The map manager fires a PropertyChangeEvent each time the active map changes. To keep track of the active map, you can register a property change listener with the map manager:

ILcyLucyEnv lucy = ...;

//Retrieve the map manager
TLcyCombinedMapManager mapManager = lucy.getCombinedMapManager();

//Register the property change listener
mapManager.addPropertyChangeListener(evt -> {
    String propertyName = evt.getPropertyName();
      ILcyGenericMapComponent activeMap = (ILcyGenericMapComponent) evt.getNewValue();

      if(activeMap != null){
        //do something with the map