I was involved in the recording and production for a top 40 producer, and can confirm that there was autotune on every single vocal track that left the studio.
Here are a few that I was in the room when the artist was recording, and can confirm pitch correction:
The first one has that metallic sound that is a dead give away. First falsetto is quieter than the second one, and you can really hear as he increases the loudness of his voice, the metallic kicks in https://youtu.be/450p7goxZqg?t=1m27s
Second one has a "Cher moment" almost straight away, just after "wandering the desert a thousand days" the following "mmmm" has a glissando between two notes where we clearly hear the hard edge on what I assume is an auto tune lookahead. I don't actually know how they work, I just assume there's a lookahead for the next note approximation which makes glissandos sound funny. https://youtu.be/M8uPvX2te0I?t=31s
The last one I can't really fault for too much autotune, more a lack of it. The bridge is especially intense https://youtu.be/E0oyglKjbFQ?t=1m51s
Say the singer loses the pitch slightly for half a second on a held note. If that fluctuation is corrected, what auditory information could be left for you to detect the modification?
I believe I hear pitch correction when it's obviously used, and it's a lot. Pretty much most of the "top forty" pop pablum from the last 20 years. I believe there is pitch correction that I don't spot: the "dark matter" of pitch correction that is done less cheesily.
The worst of it sounds almost like packet loss concealment in a G.722 voice stream: the sustained part of a vocal note basically sounding synthesized.
