When you’re confined to a hospital bed for several months the way I was last summer, you read a lot of books. During my 2+ months of medical incarceration, I read pretty much everything loaded onto my trusty old Kindle (over 200 titles.)
Often I didn’t remember how the books got there. I’d just start reading where Amazon plunked me down.
But it’s amazing how soon I could tell a book was an amateur effort. Sometimes I stuck around to read the rest of the book for story, but often I just clicked away.
Do note: I think it’s important to make a distinction between an amateur self-published book and one from a professional indie author. Few people can tell the difference between a book self-published by a pro and one from a traditional publishing house.
But amateurs tend to fly some red flags early on to let the reader know they’re kind of winging the whole publishing thing.