Is there anything I can use that will give me the "probable encoding" for a file / string / whatever?

I suspect that Encode::Guess might be your best bet. See

perldoc Encode::Guess
[download]

I am not sure how well it deals with DOS code page encodings though, especially if there may be several possibilities.

But how did he know to use 'cp437'?

I went to Google, entered: DOS code page, and pressed "I'm feeling lucky!" ;-)