How to check that the SparkContext has been stopped?

If you use spark 1.5, that could be done via reflection API:

boolean isStopped(SparkContext sc) throws NoSuchFieldException, IllegalAccessException {
    Field f = sc.getClass().getDeclaredField("stopped"); 
    f.setAccessible(true);
    AtomicBoolean stopped = (AtomicBoolean) f.get(sc);
    return stopped.get();
}