[Ethereum] ERC20 Token Transaction

erc-20go-ethereumjson-rpctokens

How does etherscan know that a particular transaction is an ERC20 token transfer?
If you look here: https://rinkeby.etherscan.io/tx/0xa8e4aa4f533e03bd8298f393d50d4ca41dba3368493c481e386d11389008353d etherscan knows that account A is transfering account B 200 tokens. IM quite familiar with quering ethereum using JSON RPC, so IM wondering if that info is there somewhere.

Thanks

It knows because:

The contracts implement the standard ERC20 interface
The address is calling the transfer(address to, uint amount) function on the ERC20 contract
The contract logs the standard ERC20 event Transfer(address to, uint amount)

As you can see in the token tutorial, at the end of the transfer method the following code is executed:

/* Notify anyone listening that this transfer took place */
Transfer(msg.sender, _to, _value);

This emits an Event.

You can 'listen' for emitted Events using web3.js for example.

You can see your tokens on etherscan or other platforms. Here for example you can see it also with the current worth of the tokens.