It is impossible to know if a check is real unless you take it to the financial institution it came from. That financial institution can verify the check’s authenticity and provide you with cash or a cashier’s check that you can deposit into your account.
If you are unsure whether a check is valid, you should not deposit it into your account. It can take days or weeks for a check to come back as fraudulent.
If a check is returned as invalid, RCU will reverse the deposit from your account and you will be responsible for the money. This may cause you to overdraw on your account.