title: Check if string is a hash
body: <p>I'm using SHA-512 to hash my passwords (with a salt ofcourse). I don't think that what I want is possible, but let's ask anyway.</p>
  
  <p>Is there a way to check if a string is a SHA-512 (or another algorithm) hash already?</p>
  
  <p>When a user logs in, I want to do a check on his password. If it's still in plain text, it should get converted to a secure form.</p>