It's difficult to explain in words, of course. Much of it comes with increased exposure to AI art and how things look. Over time it becomes pretty intuitive. There are some obvious elements that people fall back on like the way fingers look or the connectedness of certain pieces of the art that seem disjointed.

There are also standardized "models" that are used to generate AI art, and these models oftentimes share similarities in artstyle and aesthetic based on the art they were trained off of. This is another way it can become increasingly apparent. If I had to pull up two obvious examples (and I hope this does not mess with the rules):

In the above image, you can see some of those continuity issues I mentioned. The gold around the collardoesn't make sense in how it's connected to the shirt. The shadow on her left cheek isn't cast by anything and doesn't make sense. Her collar has another gold trim around it that is out of place and doesn't continue the full circumference. The ornament itself has an inconsistent design.

Another example would be:

This is a case I wanted to include because of artstyle. This is an extremely common model for AI generated art, and as a result there are A LOT of images with this artstyle that will give it away immediately. But you can still identify some issues in this image. First, zooming in, the lips and teeth have some artifacting on them. The curve of her smile on the left side of it is twisted weirdly. There is a very common sort of "mushy/fluid" look to a lot of AI art when it comes to finer details. As if it got "blurred" while bein gcreated. Looking at her eyelashes, you can not only see some of that sort of blending I'm talking about, but you can also see how messed up the lashes themselves are.

If you are uncertain, a quick reverse search couldn't hurt using your browser's search engine! Since most of what's posted here is pulled from the internet, you should be able to backtrace the image to the source and see whether it is from an AI-related account, website, or post originally.

In addition to what Dalia said, patterns also tend to help. Its why people like to point at hands, but anything that falls under a repeated pattern is probably something to look at like teeth, the lashes like Dalia pointed out, or really any other repeated pattern. The models tend to not do well with being consistent for when to start or stop these patterns.

they usually have identifiable art styles that I've seen people call "too polished"

Usually hands or inconsistencies across their works are good indicators

if its a fan art pay close attention to the eyes, 9/10 times the ai gets the eye color completely wrong (changes brown eyes to blue for example)

if you are familiar with lighting of anatomy, that's also a good indicator since the ai tends to add light from non existent light sources

In addition to what others have said, I'll also point out how if someone posts multiple images that could be described in the same way but don't share any work between them (ergo, it looks like the entire thing was redrawn from scratch), that's a huge red flag. Artists may make variants, but when they do, they usually do it by sharing some base. For instance, they may use the same general drawing but put different expressions or pieces of clothing on the character. They rarely ever draw the same thing twice from scratch but slightly differently