If you were never married to the child's father then the only way that the noncustodial parent can claim the child is by obtaining a signed form 8332 from the custodial parent. A divorce decree (or legal separation order in states that have that provision in their law) is the only document the IRS will recognize as granting a non-custodial parent the right to claim a dependent without a signed form 8332.
The reality is that if the child's father claims the child and you do not the IRS may not question it. That does not mean it is legal, and the proper way to do it is with the form 8332.

