If my ex claimed my children as dependents will a prompt let me know someone has already claimed them or will I have to wait until IRS denies it.

[WillK · Accepted solution]

If your ex claimed the children and already filed his return, when you go to e-file, the IRS will reject your return and let you know that those social security numbers already have been claimed. If that is the case, you will need to remove the children from your return. 

[Hal_Al]

If your ex claimed the children and already filed his return, when you go to e-file, the IRS will reject your return and let you know that those social security numbers already have been claimed. If that is the case, you need to print and mail your return, if you want to claim the kids.

It's not "first come first served". The IRS will resolve the conflict.

[Hal_Al]

If someone else claimed your child inappropriately, and if they file first, your return will be rejected if e-filed. You would then need to file a return on paper, claiming the child as  appropriate. The IRS will process your return and send you your refund, in the normal time. Shortly (up to a year) thereafter, you'll receive a letter from the IRS, stating that your child was claimed on another return. It will tell you that if you made a mistake to file an amended return and if you didn't make a mistake to do nothing. The other party will get the same letter you did. If one of you doesn't file an amended return, unclaiming the child, the next letter, from the IRS, will require you to provide proof. Be sure to reply in a timely manner.
Winner gets the tax benefits; loser gets to pay the IRS back with penalties and interest.  The custodial parent almost always wins. The non-custodial parent can only claim the child as a dependent if the custodial parent gives permission (on form 8332) or if it's spelled out in a pre 2009 divorce decree.   
<a rel="nofollow" target="_blank" href="https://www.thebalance.com/claiming-same-dependent-audit-risk-3193030">https://www.thebalance.com/cl...> (

[Hal_Al]

There is a way to split the tax benefits. For future negotiations with the other parent (and maybe even for this year) the following info may be of use:

 There is a special rule in the case of divorced & separated (including never married) parents. When the non-custodial parent is claiming the child as a dependent/exemption/child tax credit; the custodial parent is still allowed to claim the same child for Earned Income Credit, Head of Household filing status, and day care credit. This "splitting of the child" is not available to parents who lived together at any time during the last 6 months of the year; then only one of you can claim the child for any tax reasons. The tax benefits may not be split in any other manner.

Note in particular that the non-custodial parent can never claim the Earned Income Credit, Head of Household filing status or the day care credit, based on that child, even when the custodial parent has released the exemption to him.
 So, it's good idea to let the other parent know that you will be claiming those items, as many first time divorced parents are not aware of this rule and may try to claim those items, which will cause the IRS to send out letters.
Ref: <a rel="nofollow" target="_blank" href="http://www.irs.gov/publications/p17/ch03.html#en_US_2014_publink1000170897">http://www.irs.gov/publi...> Scroll down to "Children of divorced or separated parents (or parents who live apart)"