lookup.rb

name = "readline" # (the one you used in your require)

extensions = %w(rb so dll)
extensions.each do |ext|
  expanded_path = $LOAD_PATH.map { |p| File.expand_path(p) }
  result = catch(:found) do
    expanded_path.each do |p|
      f = "#{name}.#{ext}"
      if File.exist? File.expand_path f, p
        throw :found, [f, p]
        # found, use this
      end
    end
    nil
  end

  if result then
    # The found file is then obtained a realpath (symlinks resolved)
    # before adding to $LOADED_FEATURES
    real = File.realpath(result[0], result[1])
    puts "found: #{real}"
    break
  end
end