| I am afraid that you really won't get any luck with any IRS agency due to the strict confidentiality rule.
The only 2 choices that I can see are one that you should consider having your lawyer directly contact her lawyer and getting the information that you need.
Alternatively, you could claim the daughter on your tax return and then let the chips fall where they can, and this would force your ex to discuss with you who should be claiming this deduction. If you do not get a notice from the IRS, then you know your Ex may not have be claiming the daughter as a dependent. And, if you do get a notice disallowing the dependency claim on your tax return, you will know that the the ex has has been claiming the daughter! | 
