Return list of words ignored because they are missing from the vocabulary

If you use a grammar, you will get a bunch of messages like this one:
UpdateGrammarFst():recognizer.cc:308) Ignoring word missing in vocabulary: 'snivel'
UpdateGrammarFst():recognizer.cc:308) Ignoring word missing in vocabulary: 'classifier'

Is there currently some way to directly check which words are flagged as missing? Or should I modify UpdateGrammarFst, something like this?

while (getline(ss, token, ' ')) {
int32 id = model_->word_syms_->Find(token);
if (id == kNoSymbol) {
KALDI_WARN << "Ignoring word missing in vocabulary: '" << token << "'";
missing_words.push_back(token);
} else {
sentence.push_back(id);
}
}