boolean isListening(final SignalListener listener, final Signal signal) {
    assertNotNull(signal,
        "The signal to determine whether the listener is registered listening for cannot be null!");
    return signalListeners.get(signal).contains(listener);
  }
Map used to register SignalListeners with SignalHandlers...
geode-core/src/main/java/org/apache/geode/internal/process/signal/AbstractSignalNotificationHandler.java