I want to find all fonts that support Polish accented characters (ąćęłńóśżź).

First we will use printf to check the unicode number for a character:

printf "%x" \'ą

Then use fc-list command with “charset” query:

fc-list ':charset=17a'

/usr/share/fonts/truetype/lato/Lato-Medium.ttf: Lato,Lato Medium:style=Medium,Regular
/usr/share/fonts/truetype/lato/Lato-SemiboldItalic.ttf: Lato,Lato Semibold:style=Semibold Italic,Italic
/usr/share/texmf/fonts/opentype/public/lm/lmmonolt10-oblique.otf: Latin Modern Mono Light,LM Mono Light 10:style=10 Oblique,Italic
/usr/share/fonts/truetype/dejavu/DejaVuSerif-Bold.ttf: DejaVu Serif:style=Bold

To see how the font is rendered, you can use gnome-font-viewer:

gnome-font-viewer /usr/share/fonts/truetype/lato/Lato-Medium.ttf