latex How do I know what symbols/characters are available in a font package

% https://tex.stackexchange.com/questions/380775/font-table-for-opentype-truetype-fonts

documentclass{article}
usepackage{geometry}
usepackage{fontspec}
usepackage{longtable}
usepackage{array}

setlengthextrarowheight{3pt}

ExplSyntaxOn
NewDocumentCommand{OTfonttable}{m}
 {
  group_begin:
  #1
  int_gset:Nn g_fonttable_rows_int
   { int_div_truncate:nn { XeTeXlastfontchar font + 15 } { 16 } }
  int_gset:Nn g_fonttable_chunks_int
   { int_div_truncate:nn { g_fonttable_rows_int + 31 } { 32 } }
  group_end:
  fonttable_make:n { #1 }
 }

int_new:N g_fonttable_rows_int
int_new:N g_fonttable_chunks_int
seq_new:N l_fonttable_rows_seq
	l_new:N l_fonttable_font_tl

cs_new_protected:Nn fonttable_make:n
 {
  	l_set:Nn l_fonttable_font_tl { #1 }
  int_step_inline:nnnn { 0 } { 1 } { g_fonttable_chunks_int - 1 }
   {
    seq_clear:N l_fonttable_rows_seq
    int_step_inline:nnnn { 0 } { 1 } { 31 }
     {
      seq_put_right:Nx l_fonttable_rows_seq
       {
        fonttable_setup: int_to_Hex:n { ##1*32 + ####1 }x &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 0 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 1 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 2 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 3 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 4 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 5 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 6 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 7 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 8 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 9 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 10 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 11 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 12 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 13 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 14 } } &
        fonttable_char:n { int_eval:n { ##1*512 + ####1*16 + 15 } } 
       }
     }
    egin{tabular}{|r|*{16}{c|}}
    cline{2-17}
    multicolumn{1}{c|}{} &
      	tfamily 0 &
      	tfamily 1 &
      	tfamily 2 &
      	tfamily 3 &
      	tfamily 4 &
      	tfamily 5 &
      	tfamily 6 &
      	tfamily 7 &
      	tfamily 8 &
      	tfamily 9 &
      	tfamily A &
      	tfamily B &
      	tfamily C &
      	tfamily D &
      	tfamily E &
      	tfamily F \
    hline
    seq_use:Nn l_fonttable_rows_seq { \ hline } \
    hline
    end{tabular}clearpage
   }
 }

cs_new_protected:Nn fonttable_setup: { 	tfamily $vphantom{ig|}$ }

cs_new_protected:Nn fonttable_char:n
 {
  	l_use:N l_fonttable_font_tl
  iffontcharfont #1 symbol{#1} fi
 }
ExplSyntaxOff

% 
ewfontface{	est}{Old Standard}

ewfontfamily	est{sr2.ttf}
setCJKmainfont{sr2.ttf}
setCJKsansfont{sr2.ttf}
setCJKmonofont{sr2.ttf}
egin{document}

OTfonttable{	est}

end{document}