Only the custodial parent can claim the kid except if there is a court order.
If you think that you are the one who can claim the dependent, then file your tax return and claim the dependent. It you are unable to efile, then send paper return. IRS with then contact both of your and figure out who must amend the tax return and pay the tax due with interest and penalty. Your U.S. Tax Return: Child of separated or divorced parents (http://taxipay.blogspot.com/2008/03/child-of-separated-or-divorced-parents.html)