test.rb

# encoding: utf-8

require "suddenly_death"
require "ttfunk"

file = TTFunk::File.open(ARGV[0])

ARGV[1].split(//).each do |c|
  l = 0
  file.cmap.unicode.each do |u|
    begin
      l = u[c.unpack("U*").first]
      break unless l == 0
    rescue NotImplementedError
    end
  end

  next unless l == 0

  suddenly_death "表示できません"
  exit 1
end

Brushed up version:
https://github.com/tnayuki/check_ngui_localization