Actually you can: String.codepoints/1 will decode what it can and show you the code points it can’t.

iex> x = <<160, 73, 100, 111, 119, 117, 32, 84, 97, 121, 108, 111, 114, 32, 83, 60, 47,
  99, 111, 110, 116, 97, 99, 116, 45, 97, 100, 100, 114, 101, 115, 115, 62, 10>>
iex> String.codepoints x
[
  <<160>>,
  "I",
  "d",
  "o",
  "w",
  "u",
  " ",
  "T",
  "a",
  "y",
  "l",
  "o",
  "r",
  " ",
  "S",
  "<",
  "/",
  "c", 
  "o",
  "n",
  "t",
  "a",
  "c",
  "t",
  "-",
  "a",
  "d",
  "d",
  "r",
  "e",
  "s",
  "s",
  ">",
  "\n"
]

While I have very little windows experience, I would expect this error to be caused by shell being not in unicode encoding.

In general any unicode issues on windows related to the shell were solved by using the graphical iex --werl shell.

You can try to use the raw representation of the string.

iex(30)> i "하이"
...
Raw representation
  <<237, 149, 152, 236, 157, 180>>
...
iex(31)> i <<237, 149, 152, 236, 157, 180>>
Term
  "하이"
...
Raw representation
  <<237, 149, 152, 236, 157, 180>>
...
iex(32)> str = <<237, 149, 152, 236, 157, 180>>
"하이"
iex(33)>

As you can see your string has just the wrong encoding.

iex(43)> String.to_charlist(<<199, 207, 192, 204, 34, 10>>)
** (UnicodeConversionError) invalid encoding starting at <<199, 207, 192, 204, 34, 10>>
    (elixir) lib/string.ex:1801: String.to_charlist/1
iex(43)>