While there are many great answers regarding the "glyph not found" glyph, that won't help you actually detect it, as the text string in code will still have the character regardless of the font used to render it.

When you visit location /one and the server redirects you to location /two, you expect the browser’s address bar to display the redirected URL. However, Turbolinks makes requests using XMLHttpRequest, which transparently follows redirects. There’s no way for Turbolinks to tell whether a request resulted in a redirect without additional cooperation from the server. To work around this problem, send the Turbolinks-Location header in the final response to a visit that was redirected, and Turbolinks will replace the browser’s topmost history entry with the value you provide.