I do this with a bit of CSS:

/* Pay-Wall Domains */

a[href^="https://www.ft.com/"]::after,

a[href^="https://www.newscientist.com/"]::after,

a[href^="https://www.telegraph.co.uk/"]::after,

a[href^="https://www.thetimes.co.uk/"]::after,

a[href^="https://www.wsj.com/"]::after {

  color: rgba(128,0,0,0.75) !important;

  content: " $ " !important;

}

I imagine there are loads more pay-walled domains here, but these are the ones I've seen frequently...

If it's not obvious, this places a red dollar sign after any links to the listed domains.