We believe that my fiance's baby momma, is on weed, and doing it around the child. How can we prove it?

My fiance and the mother of his child have a custody aggreement. But now the child is screaming when we have to give her back to her mother, and dosen't want to go. She has had mrsa 3 times on her bottom, and we don't know how. About a week ago when we picked her up, she smelled of nothing but piss, and weed. As soon as we got her, I made sure that i gave her a bath as soon as i could. We believe that the mother is doing drugs but we arn't 100% sure. Today when the mother picked her up, there was a know drug dealer/ pot head sitting in the passanger seat. And the mothers boyfriend just recently got out of jail on drug charges. What i want to know is how would we go about getting the little girl drug tested, and how can we prove that in court, to prove her unfit, that way we can full custody?