Plain text is, well, plain. It does not provide any standard possibility to specify its charset. If the text document uses a Unicode Transformation Format like UTF-8 or UTF-32LE, the used charset may be indicated by a Byte Order Mark ("BOM"). However, using a BOM with UTF-8 is officially not a recommended practice. Even using byte order marks won't help if dealing with documents encoded using other character sets like "Windows-1252", "ISO-8859-1" or "KOI8-R".

There are dozens of charset converters available that allow to transform a text from one character encoding to another with ease. If the document's charset is already known, using one of these tools is sufficient.