class String

def is_utf8?

iso_str.is_utf8? # => false
utf_8_str.is_utf8? # => true

iso_str = "some string".encode "ISO-8859-1"
utf_8_str = "some string".encode "UTF-8"

Returns +true+ if string has utf_8 encoding. 
def is_utf8?
  case encoding
  when Encoding::UTF_8, Encoding::US_ASCII
    valid_encoding?
  when Encoding::ASCII_8BIT
    dup.force_encoding(Encoding::UTF_8).valid_encoding?
  else
    false
  end
end