Regular Ubuntu (and variants) repositories include (at least with current LTS release 20.04) programs font-viewer
and gnome-font-viewers
.
Both can be installed using regular package tools, including apt install
as root, and called from your file manager through usual mechanisms, to preview font files from any directory.
They can also be called from command-line:
/usr/libexec/font-manager/font-viewer /path/to/font/file.ttf
gnome-font-viewer /path/to/font/file.ttf
Incidentally, I happen to have encountered some fonts (partial encoding of a subset of a character set, extracted from a PDF file) that display well in font-viewer
and show all characters substituted in gnome-font-viewer
(and same for the PDF file in most PDF viewers).