Python open(“x”, “r”) function, how do I know or control which encoding the file is supposed to have?

If a python script uses the open("filename", "r") function to open, and subsequently read, the contents of a text file, how can I tell which encoding this file is supposed to have?

Note that since I'm executing this script from my own program, if there is any way to control this through environment variables, then that is good enough for me.

This is Python 2.7 by the way.

The code in question comes from Mercurial, it can be given a list of files to, say, add to the repository, through a file on disk, instead of passing them on the command line.

So basically, instead of this:

hg add A B C

I can write out A, B and C to a file, with newlines between each, and then execute the following:

hg add listfile:input.txt

The code that ends up reading this file is this:

files = open(name, 'r').read().split(delimiter)

Hence my question. The answer I was given on IRC when I asked which encoding I should use was this:

it is the same encoding than the one you use on command line when passing a file argument

I take this to mean that it is the same encoding I "use" when I execute Mercurial (hg). Since I have no idea which encoding that is, I just give everything to the .NET Process object, I ask here.