but I need to have them in UTF-8 or ANSI.

Assuming the XML file is valid — and, since you posted this in a thread where I complained that people often produce invalid XML, that's not necessarily true — I think you can use XSLT, with an identity transform and thus make it produce XML in any encoding you like.

The question is: is there any unix/linux command that tells me the encoding format of an xml file?

Uh? Do you still need it, then? Anyway, if you don't mind a solution involving Perl, then Encode::Guess might do the trick.