You can also check what other works the user has done. In the case of your post, that user has only starting uploading works this week and they all have that AI generated look. So that's a strong tell there that all their stuff is AI generated. But also you can view the metadata of the file as the AI generators will state themselves as the source and software. You can do that from danbooru after uploading by clicking the arrows next to the Size info which will take you to here. Software is NovelAI and source is Stable Diffusion; therefore, it's AI generated. Now this isn't a failproof way to detect AI works either as a determined person could edit the metadata themselves or it might get stripped by the site they upload to. But no one is going to edit the metadata to make it seem like it was AI generated, so the chance of an image being falsely flagged this way is very low.
I can't believe I missed something as obvious as "artists" creating new Pixiv accounts and uploading loads of art in only a few days.
Now while the metadata is an excellent way to check, the biggest concern I have with that is as you said:
"You can do that from Danbooru after uploading..."
My concern is figuring it out before the upload. I did a mock upload of a different AI-generated image to see if I could check the metadata during an upload, and I can't find anything to help me with that. Asking in topic #22285 is well and good, but I wish there was a reliable method integrated into the upload process - say, it checks the metadata automatically and brings up an alert stating that the image is AI-generated. Of course I know this phenomenon is literally in its infancy, so it's too early to demand for Danbooru to implement such a change, if it can even happen. It's just a shame that Pixiv isn't as reliable as I'd like with filtering AI-generated art, which means having to train my eyes to spot a new type of art that is banned here amongst everything else like poor quality, banned artists, and objectionable content.
Still, I appreciate all of the suggestions, everyone. Like I said, I'm doing my absolute best to avoid uploading AI-generated art. As post #5749360 and post #5748912 indicate though, don't be shocked if I make such a mistake again in the near future.