How to determine encoding table of a text file

You can't reliably detect the encoding from a textfile - what you can do is make an educated guess by searching for a non-ascii char and trying to determine if it is a unicode combination that makes sens in the languages you are parsing.

See this question and the selected answer. There’s no sure-fire way of doing it. At most, you can rule things out. The UTF encodings you’re unlikely to get false positives on, but the 8-bit encodings are tough, especially if you don’t know the starting language. No tool out there currently handles all the common 8-bit encodings from Macs, Windows, Unix, but the selected answer provides an algorithmic approach that should work adequately for a certain subset of encodings.

If you're on Linux, try file -i filename.txt.

$ file -i vol34.tex 
vol34.tex: text/x-tex; charset=us-ascii

For reference, here is my environment:

$ which file
/usr/bin/file
$ file --version
file-5.09
magic file from /etc/magic:/usr/share/misc/magic

Some file versions (e.g. file-5.04 on OS X/macOS) have slightly different command-line switches:

$ file -I vol34.tex 
vol34.tex: text/x-tex; charset=us-ascii
$ file --mime vol34.tex
vol34.tex: text/x-tex; charset=us-ascii

Also, have a look here.

Open the file with Notepad++ and will see on the right down corner the encoding table name. And in the menu encoding you can change the encoding table and save the file.