No magic involved, it is training, proper equipment, competency, knowledge of systems and experience. However you seem to resist the idea of doing something that would be legally required of you and should be common sense.
You are not competent to undert 8ake this kind of testing and would be working outside of EAWR, H&S act, as well as BS7671. However you came here for advice and the advice to get an electrician may save your life or others life or injury to you or others. Personally I am aghast at your cavalier attitude to safety, shown by the above reply. You say you are being barbecued because you asked, you rightly deserve flaming with such an obdurate resistance to what is sound and wise advice. But hey what do we know, I wonder why you bothered asking if you don't like the simple advice to get someone qualified and legal to do such work. What would it cost? Is it the money stopping you or just stubbornness. I am concerned genuinely at your approach!