MIL and meth

How do we go about asking my mil if she’s smoking meth again? She’s supposed to move in with us and I’m concerned. She’s not like a full blown junkie but there’s things that make her seem like she’s on it. Her teeth are randomly falling out and she has nasty sores. I don’t want to be mean but my kids come first and she needs to know that if she’s using again she needs to find somewhere else to go. My dad is a narcissist and we recently stopped speaking to him and allowing him to be around our kids but he has said he suspected she’s on it too. I know he will call dcf in a heartbeat if he hears she’s living with us. What can I do? Should I call dcf myself and hope they tell her she can’t be there with the children in the home or do I tell her myself? My fiancé and his mom are very close and I don’t want this to ruin our relationship.