For this, you have to select “Labs” after selecting “Settings”. Then, you have to activate “Authentication icon for verified senders”. It will allow you to figure out legitimate email.
This is a good way to know if it is a legit email as you see a key symbol in the left side of the sender name, then it is a legit email. If you don’t see key symbol in the left side of the sender name, then it is a spoof email. Check out how email frauds are undertaken as knowledge is power and prevention is the best care.