The best and simplest way is to use console command (this command should be available for all ubuntu-based distributions) :
fc-list
Example output (stripped) :
/usr/share/fonts/truetype/lato/Lato-Medium.ttf: Lato,Lato Medium:style=Medium,Regular
/usr/share/fonts/truetype/tlwg/TlwgTypo-Bold.ttf: Tlwg Typo:style=Bold
/usr/share/fonts/truetype/lato/Lato-SemiboldItalic.ttf: Lato,Lato Semibold:style=Semibold Italic,Italic
/usr/share/fonts/truetype/dejavu/DejaVuSerif-Bold.ttf: DejaVu Serif:style=Bold
/usr/share/fonts/truetype/noto/NotoSansThai-Regular.ttf: Noto Sans Thai:style=Regular
...
From MAN :
fc-list - list available fonts
To check all available options use MAN (manual): man fc-list