Да, UTF8 не е еднобайтово кодиране, така е. А за евристиката не си прав.

Извадка от Win32 API help-a:

IsTextUnicode
The IsTextUnicode function determines whether a buffer is likely to contain a form of Unicode text. The function uses various statistical and deterministic methods to make its determination, under the control of flags passed via lpi. When the function returns, the results of such tests are reported via lpi.

BOOL IsTextUnicode(

  CONST VOID* pBuffer, // input buffer to be examined

  int cb,                // size of input buffer

  LPINT lpi               // options

);