Where to check that a function actually did what I told it to?

Question

Consider this function intended to kill all running instances of a subprocess:

killAllFoobars()
{
    pids = getRunningFoobars();

    foreach ( pids as p ) {
        killOneFoobar(p);
    }

    return TRUE;
}

Where should I check that in fact all Foobars were killed? Should killOneFoobar() check that its Foobar is now killed? Should killAllFoobars() check that there are no more running Foobars before returning TRUE (seems a bit disingenuous to return TRUE without checking). Should the function calling killAllFoobars() check?

Consider that the check is an expensive operation, so should not be done 'just whenever' but rather only when necessary.