A professional relationship
Testing must take place within the context of a professional relationship. This is a requirement laid down by the code of ethical conduct for the profession of Psychology, regulation R717 of the Health Professions Act. You have a right to know who is testing you and whether they are qualified to do this. The person who is testing you should identify themselves to you and say whether they are a psychologist, psychometrist or registered counsellor. These registered professionals are issued with a blue card which is proof of their registration. You may ask to see this card or ask them for their professional registration number, which you can verify online at the HPCSA website if you want to.
If you are being tested via the Internet, it is much more difficult to establish a professional relationship. You should at least know who is in charge of the testing process and there should have been some communication between yourself and this person. You should know how to get help or explanations relating to the process of completing the test.