That is a tough one. It is legal in many states. Frankly, I don't care if an employee smokes weed on Saturday night (assuming bit is legal) any more than I care of they get drunk. Their time...both are legal.
The challenge with marijuana testing is, I don't think there is a test that can distinguish whether they had it Saturday night or Monday morning on their drive in.
Over the years I've learned more than a bit about drug testing, its an interesting industry. Alcohol is really the only drug that has a street level test that detects actual impairment. At a blood level, they can test for anything and everything as well as give you exact levels of the specific component being sought after. However, blood testing requires much more training and generally cannot be done at street level, the perpetrator in question needs to be brought to a medical facility, then the blood sample must be run through the correct system with corresponding baseline samples.
At a street level( aka doesn't need medical training for administration), you basically have urine tests, which are EMIT-based. EMIT stands for Enzyme Multiplied Immunoassay Technique. Basically there is a compound in the drug test strip that selectively binds to a component of the drug being tested. This selective binding is what changes the color/causes the strip to show up. If there's not enough of the compound present, nothing presents. The problem with EMIT tests is that they don't specifically detect impairment, especially when it comes to marijuana. The EMIT test for marijuana specifically looks for the carboxylate of THC, it's breakdown product that is no longer psychoactive. Basically, the EMIT says yes, this person has smoked sometime in the past.
Now we get into the problem of dwell time. THC and its carboxylate are both fat soluble molecules, your body fat "likes" holding onto said molecules. I have seen someone test positive for marijuana on a job screen almost three months after they quit(and yes, I know they actually quit and didn't cheat, lol). Many other drugs have water soluble molecules, your body flushes those out quickly. Look at the reports of meth users drinking urine of other meth users, or the Laplanders who discovered amanita muscaria's hallucinogenic effects via reindeer urine.
