My wife is saying that one of our children that she is allowed to claim via our divorce decree, I claimed. Where should I look to find out who claimed him?

She is saying that the child who lived with me 12 months but she is allowed to claim via our divorce decree, somehow I claimed him.  I do remember putting the info that he lived with me  for 12 months but that he was to be claimed by someone else via an agreement.  So now I don't know where to look to see if I messed something up or not?  She is saying her taxes got kicked back because I already claimed him.