Find the family name of the font and run pdflatex nfssfont
from the command line.
Hit return at the first prompt, then the encoding you wish to test, the font family name and then the other font specs (just hit return if you want the proposed default). At the last *
prompt type \bigtest\bye
and return.
Here's the console output
> pdflatex nfssfont
This is pdfTeX, Version 3.14159265-2.6-1.40.19 (TeX Live 2018) (preloaded format=pdflatex)
restricted \write18 enabled.
entering extended mode
(/usr/local/texlive/2018/texmf-dist/tex/latex/base/nfssfont.tex
LaTeX2e <2018-12-01>
(/usr/local/texlive/2018/texmf-dist/tex/latex/base/article.cls
Document Class: article 2018/09/03 v1.4i Standard LaTeX document class
(/usr/local/texlive/2018/texmf-dist/tex/latex/base/size10.clo))
No auxiliary output files.
**********************************************
* NFSS font test program version <v2.2d>
*
* Follow the instructions
**********************************************
Input external font name, e.g., cmr10
(or <enter> for NFSS classification of font):
\currfontname=
*** NFSS classification ***
Font encoding [T1]:
\encoding=T1
(/usr/local/texlive/2018/texmf-dist/tex/latex/base/t1enc.def)
Font family [cmr]:
\family=EBGaramond-LF
Font series [m]:
\series=
Font shape [n]:
\shape=
Font size [10pt]:
\size=
(/usr/local/texlive/2018/texmf-dist/tex/latex/ebgaramond/T1EBGaramond-LF.fd)
Now type a test command (\help for help):)
*\bigtest\bye
[1{/usr/local/texlive/2018/texmf-var/fonts/map/pdftex/updmap/pdftex.map}]
[2] [3] [4]{/usr/local/texlive/2018/texmf-dist/fonts/enc/dvips/ebgaramond/ebgm_
xxcxg5.enc}</usr/local/texlive/2018/texmf-dist/fonts/type1/public/ebgaramond/EB
Garamond-Regular.pfb></usr/local/texlive/2018/texmf-dist/fonts/type1/public/ams
fonts/cm/cmr10.pfb></usr/local/texlive/2018/texmf-dist/fonts/type1/public/amsfo
nts/cm/cmr7.pfb></usr/local/texlive/2018/texmf-dist/fonts/type1/public/amsfonts
/cm/cmti10.pfb></usr/local/texlive/2018/texmf-dist/fonts/type1/public/amsfonts/
cm/cmtt10.pfb>
Output written on nfssfont.pdf (4 pages, 199258 bytes).
Transcript written on nfssfont.log.
and here's an excerpt from page 2:

\specimen{}
could have some options likemono
for mono font as you said, or it could force the display of all a specific laguage characters. Like if I ask\speciment[mono,french,cyrl]{Anonymous Pro}
it will display all programming characters and also all extend french character available (like éèêç…) and all cyrillics character of this font. But, it’s not my ask for the moment. If this command could only print all characters of a font and a lipsum text sample, it would be great. – fauve Dec 22 '18 at 21:38fontspec
and lualatex, but no luck yet. – Thérèse Dec 23 '18 at 1:29