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.