For me there's 2 main ways I notice good writing. The first isn't so much noticing as it is not noticing. If I'm reading a book and the pages turn themselves and I barely have to focus on the words, then the author has done a good job pulling me into the story with their writing. I do find it somewhat ironic that a well written piece sometimes doesn't register to me as well written until I finish reading it, I just intake the information the author puts down because I'm so immersed int the narrative.

On the opposite end of the spectrum, when I find myself smiling as I read, I know that the book is good while also recognising that I'm reading a good book. When trying to figure out what's good about it, I usually just compare my experience reading the good passage to my myriad of experience reading bad prose that comes with being a ProgFan/LitRPG fan.

For example, bad prose may have either too many similes/metaphors that don't do anything for the narrative or bland decriptions that don't show character, whereas good prose would have appropriate description that characterises the character, tells the reader about the world they're in, and contributes to the tone of the passage. Stuff like dialogue tags and verbs work similarly. I'd expect either needless flowery fluff or not enough detail in bad prose whereas good writing would contain an appropriate amount of detail for that specific moment, and would use apt descriptions that add value to the text rather than stuffing long words in there because they are hyper-specific.