One question that gets asked again and again is: “How can I tell whether someone is using a screen reader or some other assistive device when browsing my site?” The short answer is that you cannot. You can only identify what the browser and operating system is from the USER_AGENT HTTP header. However, there is a rather cunning trick that you can use if you have Flash MX to hand.
Create a movie that does nothing, and use the Accessibility.isActive() method to identify if an assistive device is being used. Depending on the result you can use this information however you see fit - perhaps directing the user to a more accessible version of your site. Of course, you cannot tell what particular piece of assistive technology is being used, but this is still a useful tool to have at your disposal.