For the most part I can actually tell, but it also depends on the style of the art. A lot of anime-inspired digital images are immediately obvious - AI tends to add quite a lot of "shine" to its output, if that makes sense. And it's way too clean, sterile even. And it all looks the same.
But when the art style is more minimalist or abstract, I find it genuinely difficult to notice a difference and have to start looking at the finer details, hence the mental workload comment. Often times I'll notice an eye not facing the right direction or certain lines appearing too "repetitive", something I rarely see in the works of human artists. It's difficult to explain without actual inage examples in front of me.
That's a kind of survivorship bias though; sure, 100% of AI art that you identified as AI art turned out to be AI art, but what about the ones you didn't realise were AI art? It's the unknown unknowns. Was this comment written by AI?
The success rate of AI evading detection will only increase; the issue with "too many fingers" was solved years ago, and there's probably companies actively working on avoiding AI detection already. And on detecting it. It's the new spam / anti-spam, virus / anti-virus arms race.
To me AI generated art without repeated major human interventions is almost immediately obvious. There are things it just can't do.