How to know whether a file's encoding is ansi or utf8?

[Sam Hu via Digitalmars-d-learn]

On Tuesday, 22 July 2014 at 09:50:00 UTC, Sam Hu wrote:
> Greetings!
>
> As subjected,how can I know whether a file is in UTF8 encoding 
> or ansi?
>
> Thanks for the help in advance.
>
> Regards,
> Sam

Sorry,I mean by by code,for example,when I try to read a file 
content and printed to a text control in GUI,or to console,will 
proceed differently regarding file encoding.