How can I identify whether a browser/reader is being used by someone who is visually impaired?

I have to provide a 508-compliant version of a page for the visually impaired and i开发者_StackOverflowt isn't practical to just modify the page. Thus, I have created a new "reader-compatible" version of the page for the handicapped. The question is how I can tell if someone is using a reader rather than a standard browser? Is there a "header" or "useragent" value typically used by these non-standard browsers that I can look for?

Thanks in advance!

After corresponding with a number of people who specialize in accessibility or who actually build readers for the accessibility market, the conclusion is that there is no way to tell if a visitor is using a reader. Thus, the only alternative is to ask the user before they are routed to a page that has different versions for a reader/non-reader.