I think it can be impossible to tell.

Some initial idea was that all AI art is otherwordly beautiful, mix of realism and anime, but there are instances of long-time artists with such exceptional skills having to put up whole process in order to prove that they indeed made it all by themselves. Many styles have names and if you add name of a style to prompt you will get that style-people who use AI to design patterns for notebooks or stickers or phone cases give a prompt kind of like this "cartoony cute small simple happy puppies" and they get that, with small errors on some of them that they can easily crop and fix.

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Looking at someones profolio could help determine and to see if they have community presence or artist appeared out of nowhere, also if they can't provide steps of process.