read_utf-16_with_bom.rb

text = File.open('./UTF-16-with-BOM.txt', 'rb') {|f| f.read}
case text[0..1].bytes
when "\xFE\xFF".bytes
  encoding = 'UTF-16BE'
when "\xFF\xFE".bytes
  encoding = 'UTF-16LE'
else
  encoding = 'US-ASCII'
end
puts text.encode('UTF-8', encoding)

We can also do like following:

puts File.open('./UTF-16-with-BOM.txt', 'r:bom|utf-16:utf-8') {|f| f.read}