Here's a page that talks about it. Not sure that I would trust a lawyer who uses geocities, but it does state the following:
1. In order to prove beyond a reasonable doubt that defendant was driving while intoxicated by marijuana, the state must show (1) that marijuana was present in the defendant's system while he was driving; and (2) that the marijuana resulted in a substantial deterioration or diminution of the defendant's mental faculties or physical capabilities as to make it improper for him to drive. In other words, the state must prove that marijuana was the proximate cause of defendant's behavior.
2. The state can prove DWI by marijuana - or any other CDS - by presenting expert testimony that a defendant is "under the influence" of a drug or narcotic "from the subject's conduct, physical and mental condition and the symptoms displayed." Even a nonexpert witness, such as a police officer who has had the "specific schooling and training in the field of narcotics ... if sufficiently experienced and trained may testify generally as to the observable reaction of drug users."
3. Marijuana intoxication is not a matter of common knowledge such that an inference of intoxication may be drawn solely from a lay witness's testimony respecting defendant's behavior.
http://www.geocities.com/kenvnjlaw/MarijuanaDWI.html
If they administered roadside tests and she passed, I don't understand why she was charged.