Did you read the very first result on that search?
ERC20 token should be indistinguishable from each other because it's a standard devised precisely to encourage and facilitate trading.
This is one example: https://github.com/bokkypoobah/TokenTrader/blob/master/contr...
Originally asked here: https://ethereum.stackexchange.com/questions/12757/detect-or...
You can also check with MyEtherWallet: https://ethereum.stackexchange.com/questions/21082/use-mew-f...
The short answer is that you need the contract code and ABI to check if it is a ERC20 compliant token or not but you can use http://testnet.etherscan.io/ (or the live version) to get this information.
To reach the ethereum genesis block programatically, starting from an erc20 token I give you, you need that "need the contract code" part which is centralised.
The contract that holds the ERC20 is hosted in every single node of the blockchain so it is not centralised at all.