Alcohol is close to fully legal, but it is easy to control, Scotch and Whiskey come from places, and has to be bottled and transported it doesn’t get produced in my garden, or by my front door, or in the big pot on the patio.
I can grow great pot five feet from where I am sitting right now, or even in this very room.
Once pot is legal, and the seeds are being scattered everywhere, and the plants grown everywhere, there won’t be attempts to do testing on the source of your grass, it will just become legal.
So where is the problem? If you smoke what you grow, it's up to you to do whatever quality control you want; it'll still be as illegal for an individual grower to sell or gift bad weed, just like it's illegal to distribute tainted Halloween treats; and many consumers will be willing to pay a premium to get corporate-grown weed with the attendent resources and incentives for quality control.
One male pot plant can air fertilize over many miles - resulting in a seed heavy mature female that is about useless for smoking. And because the male growing wild is selecting for different traits than the cultivated varieties, the seeds will no doubt be about useless as well.
There is a reason they call it “Alabama dirtweed”.