Regular Ubuntu (and variants) repositories include (at least with current LTS release 20.04) programs
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:
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).