I last read up on this about four years ago. I'm an analytical chemist, and I get checked every time I go to a new employer--guess when I changed jobs last? :>) There are two basic ways to check if THC is in your body. One is a biochemical test that is really fast, and the other uses more traditional chemical analysis. As of four years ago, the fast ones (like three minutes test time) were pee strips and saliva tests. The traditional method is was for pee, blood and hair....whatever, but you need a fair amount of it.
The saliva tests could tell if you'd smoked in the last 4 hours, no matter how often you smoke. They cost 10x what the pee test do, so not many organizations were using them at the time. If you hadn't injested in the last four hours, it wasn't going to see anything.
The pee test is the one you hear the 30 day warnings about for heavy smokers. It is chemically the same as the saliva test. The difference is how long THC stays in saliva, as opposed to pee.
The standard chemical testing is the least sensitive, and the most expensive. It was also regarded as lock down proof by the legal system, because it has been around longer than biochemistry. In the case of my employer, they used it only if you failed the pee strip test---to hold down costs of the testing program. Then, it was used as confirmation of the pee results. Of course they had to use both on me. I drank so much water I'd knew the tests would be "inconclusive", so they couldn't fire me, and it wasted lots of their money. Ha. Ha.
There isn't a rule as to what constitues overwhelming evidence. It's all between the courts and lawyers. Rule of thumb, though: The more experienced the cop, the more the court will trust his observations.