While there are tests to verify whether a person has marijuana in their system, there is no definitive roadside test law enforcement has to use to see "how high" a driver is. A test of this sort is still being developed to ensure road safety and responsible use.
While tests to check whether or not a person has marijuana in the system exist, they are not always accurate. The "high" could have worn off, but the substance could still be lingering in the body. There is no way to tell just yet when the marijuana was consumed.