If it really is a .doc file and written in an ASCII-compatible encoding as most English-language documents are, opening it in a hex editor (or a non-codepage-aware text editor like the Notepad on a W10 or earlier Windows machine) will show an indecipherable proprietary header followed by the text in the file, possibly with a single space or "junk" character between each letter depending on the exact version of Word and system encoding it was written with. There may be occasional additional stretches of markup junk. At the end, there will be a footer with occasional decipherable text strings like "MSWordDoc" and font names.

If you open a .docx file in such a program, you should get a typical zipfile signature: the letters "PK" at the beginning of the file, followed by a lot of gobbledegook. If you don't get that "PK", it probably isn't a .docx.

(I've looked at a lot of MS file guts, for both curiosity and information extraction purposes.)