If it starts with 0xFE 0xFF, but is otherwise UTF-8 instead of UTF-16, it's a binary file. If it starts with 0xEF 0xBB 0xBF, it's a text file with a ZERO WIDTH NO-BREAK SPACE at the start.

> If it starts with 0xFE 0xFF, but is otherwise UTF-8 instead of UTF-16, it's a binary file

Sure, but who does this? All the Microsoft tooling writes 0xEF 0xBB 0xBF if you output utf8 with a BOM.