This is the police, please pull over, we have reason to believe the driver is impaired
I didn't know you cannot smoke and drive
Excuse me mister, this device says you have been smoking weed!
I'm sorry miss, my older brother gave it to me
A product needs to be made that can collect saliva and analyze the saliva to see if there is THC in the body. This can indicate if someone smoked recently.
If that product is created efficiently and accurately, it can be patented and sold to the police force for daily use to check under age smokers and even operators of vehicles.
If the product is quite successful within the police force, it can be commercially sold. Or even supplied to teachers of schools, in case they have reason to believe under age students are smoking and coming to class impaired.