Summarize:

Many septic tanks are built out of concrete. However, the tanks are often reinforced with steel bars within the concrete. These bars will trigger a metal detector, giving you a good idea of where your septic tanks are located. You can rent a metal detector from your local hardware store, and also from any large home-supply store. It's not uncommon for the builders or previous homeowners to leave future homeowners a "clue" regarding the location of their septic tank. These typically take the shape of an arrow drawn on the foundation of the house. While the arrow will not tell you how far out from the home your septic tank is buried, it will at least give you a direction to search in. If you're not sure that you've found the tanks' location, probe with a piece of rebar instead of digging. You'll need a piece of rebar at least 4 feet (1.2m) long, and a heavy hammerpreferably a sledgehammer. Hammer the rebar vertically into the ground until it comes into contact with your concrete septic tank. You can purchase lengths of rebar and sledgehammers at a local hardware store.
Search throughout your yard with a metal detector. Look for an arrow symbol on your home's foundation. Probe suspect areas with a piece of rebar.