I was wondering about that, too. The short answer would be it requires a lot of samples and a lot of different tests...
How are forensic toxicology tests done?
At the time of the autopsy, collection of blood, urine, and tissue samples is done in preparation for the toxicology tests, says Barbarajean Magnani, PhD, MD, chairwoman of the Toxicology Resource committee for the College of American Pathologists. She is also vice-chair of the department of pathology and laboratory medicine at Tufts Medical Center, Boston.
"We collect blood from different areas, such as the femoral vein [in the leg] and heart blood," she tells WebMD. That's because the concentration of drugs can be different, she says, so comparing the concentrations can boost accuracy.
''We collect urine if there is any [in the body] and also use tissues [to test],” Magnani says.
Specimens taken for forensic toxicology testing routinely include, in addition to blood and urine, tissue samples from the liver, brain, kidney, and vitreous humor (the clear ''jelly" found in the eyeball chamber), according to information from the College of American Pathologists. Samples of the stomach contents and bile, a digestive juice secreted by the liver, are also collected routinely.
http://www.webmd.com/mental-health/addiction/features/the-truth-about-toxicology-tests