UTF-8 character encoding

[Lee]

I'm looking at
  https://cygwin.com/packaging-hint-files.html#pvr.hint
and it starts off with
  Use UTF-8 character encoding.

How do I do that and how do I check that I actually did use UTF-8
character encoding _without_ using file?

for whatever it's worth:
$ file unicode.html
unicode.html: HTML document, UTF-8 Unicode text

$ file test.c
test.c: C source, ASCII text

I used vi to create both files & I'd like to understand why file says
one is ascii & the other is utf-8

Thanks,
Lee

--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple