A common method for asserting whether a sender is a contract or EOA has been to check the code size of the sender. This check asserts that if the sender has a code size > 0 that it must be a contract and if not then it must be an EOA. For example:
function mint(uint256 amount) public {
if (msg.sender.code.length != 0) revert CallerNotEOA();
}However, as noted in the Ethereum Yellow Paper, "During initialization code execution, EXTCODESIZE on the address should return zero, which is the length of the code of the account while CODESIZE should return the length of the initialization".
This repo shows how we may exploit this logic by simply calling during creation of a new contract.
As we can see, it's important that we recognize that although we may be certain that an account with a non-zero codesize is a contract, we can't be certain that an account with a zero codesize is not a contract.