People have been asking for a way to check if a font exists or not for years now. Don't think it's ever been added. Not to fear though. Perhaps we can fix that little alias you have there...

This seems to work for me...
Code:
alias -l font.exists {
  var %chr = 32
  while (%chr < 127) {
    if (($width($chr(%chr),$1-,100) != $width($chr(%chr),$1- bold,100)) || ($height($chr(%chr),$1-,100) != $height($chr(%chr),$1- bold,100)) $&
      || ($width($chr(32),$1- bold,100) != $width($chr(32),*,100)) || ($height($chr(32),$1- bold,100) != $height($chr(32),*,100))) return 1   
    inc %chr
  }
  return 0
}