How to find out if an Ethereum address is a contract?

Question

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 cont

Answer 1

Yes you can, by using some EVM assembly code to get the address' code size:

function isContract(address addr) returns (bool) {
  uint size;
  assembly { size := extcodesize(addr) }
  return size > 0;
}