Same-length sentences
People write bumpy. Some sentences are short. Others wander on for a while.
Read something your friend wrote. Some sentences are tiny. Others go on and on with lots of commas and ideas.
Writing that is copied, generated by a computer, or made in a hurry often comes out very even. Every sentence is about the same size and shape.
This is a soft clue. Lots of careful human writers are even too. Use it together with other clues, never alone.
Spot it
“Dogs are loyal animals. They enjoy playing fetch. Many families have dogs. Dogs need daily walks.”
Four sentences, four words each, same rhythm. It sounds like a robot reading a list.
Detective move
Read it out loud. Does it sound like a person talking to you, or like a machine?