1. A computer-implemented method performed by a data processing apparatus, the method comprising: receiving audio data that corresponds to an utterance; obtaining a first transcription of the utterance that was generated using a limited speech recognizer, wherein the limited speech recognizer comprises a speech recognizer that includes a language model that is trained over a limited speech recognition vocabulary that includes one or more terms from a voice command grammar, but that includes fewer than all terms of an expanded grammar; obtaining a second transcription of the utterance that was generated using an expanded speech recognizer, wherein the expanded speech recognizer comprises a speech recognizer that includes a language model that is trained over an expanded speech recognition vocabulary that includes all of the terms of the expanded grammar; aligning the first and second transcriptions of the utterance to generate an aligned transcription; and classifying the utterance, based at least on a portion of the aligned transcription, as a voice command or a voice query.