JK,
Proving driving while ability impaired by marijuana is a very tough thing to do. (I can only speak for NY). For the most part, the PD will have to get a blood sample. That means that the driver will either have to consent, or be in an accident with physical injury which allows for a warrant for your blood. The lab will test the blood, and you will get 2 readings. One will be the active metabolite and one, for lack of a better term, will be the THC. Since THC lasts in your system for 30 days plus or minus, the lab tech has to testify whether or not the numbers equate to habitual use or recent use. Sometimes it means both which complicates matters more. The lab tech can not testify that any level is per se impairing (unlike in alchohol cases where a level has been determined to be per se intox), so the lab tech will have to testify that the levels he found in conjunction with what he knows about the operation means that the person's ability to operate was impared by the use of marijuana. As I said it is a tough thing for the prosecutor to prove.
Sorry for the long answer, but there was finally a question that I knew something about!