How can I tell if a file is recorded using synthetic (computer) or live voice?

In most cases, the narrator is displayed when the file is in audio format. If no narrator is specified, the file is most likely narrated in a synthetic voice.

While some listeners may find synthetic voices difficult to get used to, most listeners quickly become accustomed to them (and even come to prefer them), especially as synthetic voice technology become increasingly sophisticated. Synthetic voices stay clear even at fast playback speeds allowing listeners to "skim" through books. Where there is a choice between synthetic content or none at all, our users’ preference is clear.

Synthetic recordings are what make it possible for us to produce audio books quickly and upon request. Some of our network members will be recording books using live narration, and there is nothing to prevent others from doing the same thing so long as: (a) copyright clearance has been completed to produce the book in question, and (b) the recorded narrations are provided to NNELS with perpetual, unlimited distribution rights to eligible print-disabled users.