CHP believes the driver who struck and killed Tracy CHP officer Andrew Camilleri was not only driving under the influence of alcohol, but also marijuana.

With recreational cannabis becoming legal January 1, how does law enforcement know if someone is high behind the wheel?

For more than 10 years, Adam Baressi has been on patrol, looking for those who shouldn't be on the roads.

“It really grasps our attention when someone is driving erratic or unsafe manner," said Adam Baressi.

So how do officers know if that erratic driver is impaired from marijuana?

There are no breathalyzer tests for pot, so officers must rely on other indicators.

“Well one of the things is red watery eyes," said CHP Officer Oscar Chavez.

Chavez has been a CHP Drug Enforcement Evaluator or DRE for 10 years.

He is specially trained to identify drivers impaired by drugs, not just alcohol.

“One of the things they might look at is an elevated pulse, maybe some dilated pupils," said Chavez.

A patrol officer can request a DRE officer to assist in determining if a driver is impaired. They may take the driver's blood pressure.

There is also the obvious smell of marijuana, but there are edibles with no smell.

A field sobriety test is always an option.

“At that time we are looking for any divided attention or impairment to see if they can operate a vehicle safely," says Chavez.

With the new law comes new concerns for law enforcement.

One in particular: new experimental users getting behind the wheel.

“They might not know how much THC levels are in the actual cannabis themselves whether its edible or smoking it," said Chavez. "We are concerned those people might try it. We do caution them not to get behind the wheel."

Bottom line, the message from law enforcement: don't drink or get high and drive or you will risk not only your own life, but others around you.

“Ultimately if we can change the behavior of those driving on our roadways in a safe manner, that's ultimately what we are here to do," said Barresi.

While statistics are kept for DUI involving alcohol or drugs, there are no statistics kept right now specifically for marijuana and driving in California.