For most it's quite easy with all the crappy AI style marks. Read a few people complain are AI to get a feel, then you can recognize it in less subtle ones.

In short comments / casual threads, it's not so easy. It can 100% look like a human comment with no special marks. Having been trained to billions of Twitter, Reddit and other messages, and the short text span, makes it difficult.

If it's so good it's not recognizable, well, not much you can do.

If you can't tell, does it matter?

https://youtu.be/kaahx4hMxmw?si=dz6RoNWrkT5bZJgq