If you don't send a content-type header most browsers will do content sniffing. If you do send a content-type header as far as I know all versions of internet explorer will do sniffing unless
x-content-type-options: nosniff is set. IE should accept this header starting with IE8 (blogs.msdn.com/b/ie/archive/2008/09/02/ie8-security-part-vi-beta-2-update.aspx).
Just an idea: you might try to detect if the browser is vulnerable to content-type sniffing by serving an HTML page with an redirect as
content-type: text/plain. If you the browser gets redirected it is vulnerable (you might also check the effectiveness of the nosniff option this way). If you just include this test-page into a small and maybe invisible iframe you can do the check without affecting the rest of the website.