What I'm saying is that during the growing a chemical should be added to the soil that can be detected in a person who smokes the pot that is produced. However, that chemical should also be metabolized by the users body at a reasonably predictable rate, like alcohol is. Ideally it would be something we could put on a scale and say a 10 means pot was consumed within the hour, a 9 means it's been 2-3 hours, an 8 equals 4-6 hours and so on. That way if you get pulled over for suspected driving under the influence of THC a test could be administered and if you were an 8 or higher you get charged with DUI.
The natural gas that is pumped into your house smells like rotten eggs. Natural gas doesn't naturally smell that way, it's actually odorless. A chemical dopant (sulfur) is added by the gas company so that if there is a gas leak you'll know it and get out of your house. Similar principle.