How did writers spot grammatical errors and use correct symbols before AI writing tools?

The same way we do now: by having a deep and intuitive understanding of the language and its grammar.

Most experienced authors do not use AI writing tools, because they actually make our writing worse. Case in point: in the previous paragraph, autocorrect changed “its" to “it’s” and I had to change it back. I'm also currently in a feud with my spell checker (which I would turn off it I knew how) because for some reason every so often it thinks I've switched to a foreign language and will tell me that my English-language words are misspelled.

And then, when I do make a mistake, often the grammar checker won't even catch it. Instead I'll see it on the third read-through as I'm looking for a particular piece of information so I don't contradict myself in a later book.