There are many possible ways to achieve this. One possible quick and dirty solution is to check if a ERC20 function exists on the contract address by calling the following:
A non-ERC20 will return a 'null' 0x hex response whereas an ERC20 will give you a 32byte uint, in this case 0 but if you provide an address in the data then it will give you the actual token balance for that address.
This is not a guaranteed way of determining a contract is ERC20 since other contracts may expose the same function, however it is a quick and easy check. You could add additional calls on totalSupply() etc. for more confirmation.