If fonts are handled by fontconfig then they are defined in /etc/fonts.
It seems the command fc-match does the mapping from ‘serif’, ‘sans-serif’, etc. to the actual fonts:
$ fc-match sans-serif
Vera.ttf: "Bitstream Vera Sans" "Roman"
$ fc-match monospace
VeraMono.ttf: "Bitstream Vera Sans Mono" "Roman"