how to determine if my dependent has been accidentally claimed

I am continuously getting rejected for a problem with my dependent I'm claiming I have

2 kids but one of them have for a fact has been claimed by the dad and the other one is supposed to be claimed by me. when the dad did his taxes he put both kids but only claimed one as dependent and the other one as non dependent. I did the exact same thing but the other way around claiming the kid he did not, it kept getting rejected. I am just confused I took off both kids on mine and and just put the one kid which then the return went down AND still got rejected so im in need of help im not understanding