I would like to use special mark symbols not only in
window-system but also in
(setq gnus-summary-to-prefix (if window-system "▒ " "==> "))
What check I should use?
It's probably not just about whether Unicode is available but also whether a given font supports a particular Unicode character.
I'm not sure that it helps for non-graphic (i.e., console/terminal) Emacs, but for a graphic display you can use
char-displayable-p is an autoloaded Lisp function in `mule-util.el'. (char-displayable-p CHAR) Return non-nil if we should be able to display CHAR. On a multi-font display, the test is only whether there is an appropriate font from the selected frame's fontset to display CHAR's charset in general. Since fonts may be specified on a per-character basis, this may not be accurate.