If you have the name of the font file, then you could just perform a quick check against the
C:\Windows\Fonts
directory and see if the file already exists there.
If I remember correctly, any font file placed in this folder will automatically get added to the registry so it is a valid way to check if the font is installed.
However, if you need to use the registry, you might find the
EnumFontFamiliesEx[
^] interface helpful.