How can I detect the encoding of a referring page?

I have a page that is charset=windows-1255 that pulls an external JavaScript file. I need this JavaScript file to be able to detect what encoding the first page is in and display itself accordingly (have the JavaScript file dynamically be in the same encoding as the page that it is in).

I'm guessing that Google Adsense do this since you can paste your Google Adsense code in pages with different encodings and the Google ads will be displayed properly (and even display ads in different languages).

TIA!