You may not need a plumber to find your septic tank. Perhaps you can find it yourself. The first question, even before how, might be “why”. Why might you need to find your septic tank? Well, a wise person may be wanting to do some maintenance, or a thoughtful, shrewd, or kind person might want to provide documentation of the location of the septic tank to a future buyer, or perhaps an inheriting descendant.
But how do you locate a septic tank, if you need to? You can try probing the ground with a steel rod, or you can buy a locator that works by detecting the metal that is used in septic tank construction. You may find something other than the tank, or you might find nothing at all. If your search is fruitless, or you find something unexpected, you might want to call a professional. A plumbing and septic system professional can work wonders with simple tools. Your local plumber is the person to call. They will know the code, the history, and the best approach to fixing your plumbing system.