How do know if you over contributed to a Roth IRA?

How do you know if you overcontributed to a Roth IRA? I have not overcontributed to a Roth IRA, but my income has been increasing and soon I will not be able to contribute to a Roth IRA. If you accidentally, overcontribute to a Roth IRA who notifies you of this? How do people find out they over contribute to their Roth IRA accounts? How are you notified of the error?