title: How to find out if an Ethereum address is a contract?
body: <p>An address in Solidity can be an account or a contract (or other things, such as a transaction). When I have a variable x, holding an address, how can I test if it is a contract or not?</p>
  
  <p>(Yes, I've read the <a href="http://solidity.readthedocs.io/en/latest/types.html" rel="noreferrer">chapter on types</a> in the doc)</p>