The tapes have to be reviewed and carefully listened to several times. The problem with that is that if you listen to anything long enough you can begin to hear things. You also have to be able to weed out all the background noises such as passing cars, barking dogs, the wind, etc. There's also the nature power of suggestion. You may want to give the tapes to a trustworthy, impartial listener.
Ghost hunters have the tendency to hear what they want to hear from the tapes.
They know what questions were asked and have an idea of what answers to expect. Because the technique's authenticity is impossible to prove no tape, regardless of what's recorded on it, can be considered hard evidence. Critics have a valid point when they say that the sounds or voices recorded are either natural noises or hoaxes. There's no way to prove that they're wrong. The small tape recorders are fine for witness interviews but not for EVP recordings.