Fasold (Fasold 1984, Sociolinguistics of Society, Ch. 6) reports on another technique: Videotape children speaking, both African-American and white, and play tapes to S's, asking them to rank the children's speech on some set of criteria. Teachers typically rank the white children's speech higher.
Then switch the sound track, i.e. play them another tape on which the voices have been switched, with white children's voices dubbed for Black children, and vice-versa. Play the same tape to the same subjects: the S's still rank the white children's speech higher.
Conclusion: ranking is extremely subjective, and S's are unable to separate the white face on the video from the voice heard.