I've taken 2 drug tests for work and nobody watched me. You go into a restroom and are alone. This was at Any Test.
Since they require only a tiny amount of urine I would think a dishonest person could put a small amount of urine in a very small container and keep it under their arm or even taped to their body to keep it somewhat warm. When I handed the sample to the technician you could tell it was mildly warm but the tech just taped it shut and put it in an envelope. Nobody took it's temperature.
Your right, I know people that fill a small liquor bottle and keep it under the breast. Comes out body temp and clean. It's just a shame that in America the average joe has to go through more invasive tests to get a job than any criminal illegal has to do to get into this Country!