\version "2.19.18"

fontExists =
#(define-void-function (parser location font-name)(string?)
   (let* ((font-file (ly:font-config-get-font-file font-name))
          (reverse-font-name (ly:ttf-ps-name font-file))
          (font-exists (string=? font-name reverse-font-name)))
     (ly:message (format "\nGiven font name: ~a" font-name))
     (ly:message (format "Determined font file: ~a" font-file))
     (ly:message (format "Actual font in that file: ~a" reverse-font-name))
     (ly:message (format "Given font is present: ~a" font-exists))))

\fontExists "Emmentaler-13"
#(display (ly:font-config-get-font-file "Emmentaler-13"))
#(newline)
#(display (ly:ttf-ps-name (ly:font-config-get-font-file "Emmentaler-13")))
#(newline)
#(newline)

\fontExists "Alegreya"
#(display (ly:font-config-get-font-file "Alegreya"))
#(newline)
#(display (ly:ttf-ps-name (ly:font-config-get-font-file "Alegreya")))
#(newline)
#(display (ly:ttf-ps-name "/usr/share/fonts/OTF/Alegreya-Regular.otf"))

\fontExists "DejaVu Sans"
#(display (ly:font-config-get-font-file "DejaVu Sans"))
#(newline)
#(display (ly:ttf-ps-name (ly:font-config-get-font-file "DejaVu Sans")))
#(newline)
#(display (ly:ttf-ps-name "/usr/share/fonts/TTF/DejaVuSans.ttf"))