How to read the encoding format of document if we don't know in which format it is written

When creating an XML document we specify the encoding format. So to know the encoding format we must first read the coding format. How can we read the coding format if don't know in which encoding the document is written.

My question doesn't concern only XML (as apparently in XML we try all the possible encoding) but I'm speaking in general. I know that in Python we specify the encoding also. And what about other specification that use the header to specify the encoding.