PHOENIX - How do you prove a marijuana DUI?

Supporters of legalizing pot say driving under the influence of marijuana would still be illegal, but law enforcement officials say there's no way to prove it.

A new breathalyzer could be the answer they're looking for.

Breathalyzers measure alcohol in parts per 1,000 but marijuana is more difficult to detect.

California-based Houndlabs says it's found a way to measure marijuana in parts per trillion.

Driving under the influence of anything is currently and those behind Proposition 205, which would legalize the drug in Arizona, say that shouldn't change.

Houndlabs expects to sell its device to police departments sometime next year for under $1,000.