Check which tokens came to the contract - ethereum

Question

Solidity. How do I check which tokens were sent to the contract? How do I check the USDT token?
contract DEX {
event Bought(uint256 amount);
event Sold(uint256 amount);
IERC20 public token;
constructor() public {
token = new ERC20Basic();
}
function buy() payable public {
uint256 amountTobuy = msg.value;
uint256 dexBalance = token.balanceOf(address(this));
require(amountTobuy > 0, "You need to send some Ether");
require(amountTobuy <= dexBalance, "Not enough tokens in the reserve");
token.transfer(msg.sender, amountTobuy);
emit Bought(amountTobuy);
}
function sell(uint256 amount) public {
require(amount > 0, "You need to sell at least some tokens");
uint256 allowance = token.allowance(msg.sender, address(this));
require(allowance >= amount, "Check the token allowance");
token.transferFrom(msg.sender, address(this), amount);
msg.sender.transfer(amount);
emit Sold(amount);
}
}

Answer 1

You're looking for the balanceOf function, you could use OpenZepplin's implementation of this as well.
uint balance = token.balanceOf(account);
For each token you want to check, you'll need the tokens address too. So for USDT, you'd need the token address of USDT. Then you could do something like (nodejs syntax):
pragma solidity ^0.6.0;
import "#openzeppelin/contracts/token/ERC20/IERC20.sol";
contract GetBalance{
constructor() public {
}
function getBalanceOfToken(address _USDT_ADDRESS) public returns (uint){
usdt = IERC20(_USDT_ADDRESS);
uint balance = usdt.balanceOf(address(this));
return balance;
}
}