There are two ways they go about it. First, if you're dealing with most employment situations, they will most likely use the cheapest test possible, which would be either a five-panel assay type test, or a strip test. These tests are cheap when bought in high volume, and companies like them because they are a cost saver. The second way it is done is by GC/MS (gas chromatography/Mass spectronomy) which is a real lab test that has to be sent out for results. Not many companies want to absorb this cost, but if you're dealing with the government or a large firm, it may be a possibility. Both types of test measure results against the 50ng/ml (50 nanograms of thc metabolites per one millileter of urine) standard. There are several ways to beat the test. Check the threads for solutions.
"Phenol contents of glands and other tissues were examined by thin layer chromatography (TLC), gas liquid chromatography-mass spectroscopy (GC-MS) and nuclear magnetic resonance spectrometry (NMR), and terpenes were analyzed by GC-MS (Hammond and Mahlberg, 1992, 1990, unpublished)."
"Maturing bracts from pistillate plants of each clone were collected for analysis. Bracts sampled were selected at 1 mm intervals from 2 mm (very young) to 9 mm (mature) in length. A minimum of three bracts of each length was processed for scanning electron microscopy (SEM) while many bracts of each length were processed for analysis by gas-liquid chromatography (GLC). Bract samples were collected at 1500 h on three consecutive days (one clone per day) during late October 1979.
Gas-liquid chromatography and scanning electron microscopy"