Basically my question is my daughter's father has custody of her and I have half legal but there's a chance she is not his I can get her back from him

Your question cannot be answered in this forum, without a full and complete review of all facts, including any and all agreements, court orders, etc.  In order to protect your rights, I suggest that you arrange a private consultation with an experienced family lawyer, in your area to discuss this matter in greater detail. If there is a court order and it is determined that the man who shares custody with you is not the father, this may create a legal justification for changing custody.  However, custody cases are fact-specific and will be based on the individual facts in your case. Most lawyers charge by the hour; therefore, you need to discuss the cost of an initial consult upfront to avoid any misunderstandings about fees.  Best of luck~