Test if string is URL encoded in PHP

Question

How can I test if a string is URL encoded?

Which of the following approaches is better?

• Search the string for characters which would be encoded, which

Answer 1

What about:

if (urldecode(trim($url)) == trim($url)) { $url_form = 'decoded'; }
  else { $url_form = 'encoded'; }

Will not work with double encoding but this is out of scope anyway I suppose?