One way is trying to sneak in a specific structure/pattern that is difficult for a human to notice when reading, like using a particular sentence length, paragraph length, or punctuation pattern. Or use certain words in the text that may not be frequently used by humans etc.

Watermarking needs to be subtle enough to be unnoticeable to opposing parties, yet distinctive enough to be detectable.

So, this is an arms race especially because detecting it and altering it based on the watermark is also fun :)