The location part of your question is easy: The byte-order mark (BOM) will be at the very beginning of the file.
When editing the file, in the bottom status bar toward the right VS Code shows you what encoding is being used for the current file:
Click it to open the command palette with the options “Reopen with encoding” and “Save with encoding”:
Click “Save with Encoding” to get a list of encodings:
Choosing an encoding saves the file with that encoding.
See also this note in the Unicode site’s FAQ about the BOM and UTF-8 files. It has no function other than to call out that the file is, in fact, UTF-8. In particular, it has no effect on the byte order (the main reason we have BOMs), because the byte order of UTF-8 is fixed.