The best way of achieving your goal is to file a motion for change of custody asking for sole physical custody of the child. In your declaration you need to specify that the reason you are requesting the change is that you have seen signs of drug addiction similar to prior occurrences and this puts the child at risk. You can't say "her behavior is that of a drugee." You need to detail what you and others see that would lead the court to a conclusion that she is addicted to a controlled substance. Then when you are before a mediator in Family Court Services you can tell the mediator that mother has a prior conviction for DUI and she is acting again like she did when she was on [name of controlled substance]. You need to ask the mediator to recommend a hair follicle test to the judge, which will carry a lot more weight than a request coming from you.
You can only obtain drug testing in the context of an ongoing custody proceeding. Further, the court lacks the power to order hair follicle testing, so don''t bother asking - they can't do it. Also, to obtain a drug test you must show by a "preponderance of the evidence" that the person seeking custody or visitation uses illegal drugs or alcohol "habitually or continually (or frequently, in the case of controlled substances)," so you need to be prepared to do that. Check out Family Code section 3041.5 (which, by the way, is set to expire and automatically "go away" on January 1, 2009 so act fast.