How does a lab actually determine the amount of meth that's found in a ua? daily meth smoker vs. someone that tested positive from second hand smoke?

There are differing methods of testing. One looks only for the detection, called the qualitative test. The other is quantitative, looking for the amount present. Most simple screening tests are looking to detect any amount of a substance. The sensitivity is the ability to detect at a certain level. Highly sensitive tests are more costly to use and may or may not be employed by the testing agency

They do it by testing for the chemical. Second hand smoke has to be very concentrated and will rarely test positive. My advice is to quit for it is very bad for your health.