Sometimes it can be very subtle. Now days almost every commercial track will have some form of pitch correction on it. For some tracks you’ll be able to hear something called artifacts on a persons voice and it’s usually easier to hear it on longer lower notes. It’s usually accompanied by a tiny digital grainy sound and warble as the software is ever so slightly bending and stretching their voice to push it further in tune. The use of a program called melodyne has made it even harder as this program aims to keep the natural inflections in the voice present while subtly bumping the pitch one way or another and is much more hands on. Autotune brand software is more of a “quick-fix” that will either automatically snap your vocal to the right pitch or more naturally slide into the right pitch depending on the settings. It’s getting harder to tell every day but almost everybody uses it now days unless they’re stubborn or don’t have access.

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It's super easy to hear auto tune, but it is getting harder as tech improves. If you want to get an ear for it, listen to older examples. T-pain was the biggest early user and once you hear his stuff, you will be able to pick it up in everything.