Tanks can be buried anywhere from 2" to over 6' depending on what sort of landscaping has taken place since it was installed.
Most tanks are buried within 50' of the home. Many have a clean-out pipe that is above ground level and close to the outside wall
Driving a steel rod into the soil until it hits the tank will tell you how deep it's buried (Be careful for pipes & electrical lines). Slumped ground, first area of snow melt in the spring, and dead lawn can also give clues to its location. If necessary, an electronic transmitter can be used to find the tank.
Every system is different & a pipe can turn & head another direction at any point. The county keeps records of most systems that drew a permit. At no charge, they will send you a map called an as-built showing the tank's location.
For Stevens County as-built records
For Spokane Country as-built records