First, you will have to print, sign and mail your return to the IRS. THen when the IRS sees that the child is claimed on 2 tax returns, both of you will be paper audited by mail. The letter you will get from the IRS will tell you what proof you need to submit. Follow the instructions in that letter to absolute perfection and it's no problem.
Basically in the end the one that qualifies to claim the child, gets it. The one that falsely claimed the child will have to pay back some of their tax refund along with interest, fines and penalties.
Do not try to "get ahead" of this with the IRS. Just mail your return and wait for the letter. Otherwise, you'll totally gum up the way the IRS does things and you'll be forever (if ever) getting any refund you are due. In a nutshell, you play the game by the IRS's rules, or you lose.
