Generally, no one will inform you of the excess contribution until it's too late to avoid one or more year's worth of excess contribution penalties.
Except in the case where you are ineligible to make a traditional IRA contribution due to you age, your broker has no way to know whether or not you are eligible to make an IRA contribution or whether or not you are eligible for a deduction for the contribution. If you make an excess contribution and do not correct it, the IRS may or may not detect it after processing the Form 5498 sent to you and to the IRS by the account custodian, generally well after you file your tax return. If the IRS doesn't detect it until later, you can end up with many year's worth of excess-contribution penalties and interest due. However, if you enter everything into TurboTax correctly, TurboTax will inform you of the excess contribution and, as long as you do this before the due date of your tax return, you'll have time to obtain a return of excess contribution before you are stuck with an excess-contribution penalty.
If you attempt to contribute more than the maximum limit to a particular IRA account (for 2017, $5,500 if under age 50, $6,500 if age 50 or over), the account custodian will often reject the contribution, but the custodian has no obligation to do so.
In summary, responsibility for avoiding or detecting and correcting an excess contribution lies with you personally.
