Corky's somewhat fictional answer:
That's not quite true.
It actually takes 45 to 60 days to be undetectable. 30 is an average, but there are a LOT of people for whom it takes longer. Hair samples can give you away months or years after you stop, depending on how long your hair is.
There are over the counter test kits in many drug stores. While not as sensitive as the ones employers and the police use, if you test positive with one you can guarantee you'll test positive if anyone pays for a high quality lab test.