For chelators which are not provocation challenge tests, only way to know if they are working at the biochemical leve is to constantly have your doctor do fractionated porphyrin (urine and/or fecal) tests for mercury. For those which are provocation challenges tests, you just follow through with the instructions of the test. Some people take DMSA and do not understand that it is a test, not just a chelator. This should help you understand more: