I would file a motion for modification, maybe an ex parte hearing where the mom would probably not be there. Show the positive test from child as evidence. Of course, you hope that the judge does not suspect you
Ask the judge to grant temp custody and order her tested.
If she is served she can just stop smoking. By the time you get to court it may be out of her system. Also, follicle only says that you have been exposed or used...does not say when.