I read this story in the 1980s, in a German anthology that was very possibly a translation of stories from Magazine of Fantasy & Science Fiction or something similar; the story has a 1950s touch to it.
The story features a (mostly fake) medium/con artist that used to claim to be able to see into the future, and make predictions. Rather to her or his surprise, they are actually able to predict something: Sometimes, a client has a mark that only the con artist can see. I remember it as a black spot somewhere on the body, but it might be something different. Whenever the mark appears, that means the person bearing it will die within a shortish period, say half a year.
The con artist makes a decent living doing predictions, when they notice that these marks appear on more and more people, at least in the city where the story is set. Nearly everybody has it, which means that something terrible is going to happen withing the next year or so, and the narrator has no idea what it is.
I think the story is told in the first person, with the narrator actually addressing the readers, trying to get this information out to somebody who might know something.
Side notes: There are lots of fairy tales where a personified death grants someone to ability to spot whether a patient will live or die, usually by a sign (or Death himself) that only that person can see. Also, there is the somewhat similar story "Otherwise Pandemonium" by Nick Hornby, whre a couple of teenager get a video cassette recorder that works without a cassette, and which shows television programmes of the future when you use the fast-forward button. (The shows are fast-forwarded, and soundless.) After some time, there are fewer and fewer of tv shows and more and more newscasts, with grave-looking people. And then, only static. So clearly, something is going to happen.