1. A method performed by data processing apparatus, the method comprising: receiving data that indicates multiple candidate transcriptions for an utterance; for each particular candidate transcription of the candidate transcriptions: receiving data indicating first search results that a search service for a first domain identifies as relevant to the particular candidate transcription; determining a first score based on the first search results that the search service for the first domain identifies as relevant to the particular candidate transcription; receiving data indicating second search results that a search service for a second domain identifies as relevant to the particular candidate transcription, the second domain being different from the first domain; determining a second score based on the second search results that the search service for the second domain identifies as relevant to the particular candidate transcription; providing, to a trained classifier, (i) the first score that is determined based on the first search results and (ii) the second score that is determined based on the second search results; and receiving, from the trained classifier, a classifier output in response to at least the first score and the second score, the classifier output indicating a likelihood that the candidate transcription is a correct transcription; and selecting from among the multiple candidate transcriptions based on the classifier outputs.