Duaneshotbabe,
There is no way to find out who claimed another person as a dependent. The only organization that knows who claims who is the IRS and they do not release the information for privacy of tax records.
You might be able to narrow down who claimed the person by thinking about who knows this person's social security number, name, and birthday.
However, what you do to get the IRS to investigate is: file your return, also claiming the dependent. If you are legally entitled to claim the dependent, write a letter to the IRS stating that someone else has claimed the dependent but is not doing so rightfully. If you can provide the IRS with copies of school records, doctor's records, birth certificate, and information of the like to prove the dependent is in your care and you have the right to claim him/her, the IRS will disallow the other person's claim and allow you to claim the dependent.
Although not always, this can be an ongoing problem in which the dependent is claimed by someone else every year. You may want to start keeping paperwork so you have proof in case you need to ask the IRS to look into the situation further.
Please let me know if I can assist you further.
Tax Advisor
5 years experience with tax preparation company, over 200 hours of tax courses taken