Is there a way to check and see if someone has already claimed your child?

My ex's wife is crazy and even thought I have already claimed my son I have with him is there a way to check and see if they have claimed him as well? I have our court order staring I get to claim our son on even years tax returns and he gets to claim him on odd years returns. So even if they have tried to claim him I have proof and will appeal but I'm hoping since I told ex I already filed he and crazy wife won't do something stupid and try to claim our son anyway. Can i check to see if they have claimed my son or not?