I don't think you'll ever, ever in your entire life, need to know this.

But.

If you are in a text file in VIM. And you need to know if it has a unicode BOM. You can type

:setlocal bomb?

and it will tell you.

If you then want to add a BOM, type
:setlocal bomb
and if it has one but you want to remove it, type
:setlocal nobomb
and in either case save.