Find it with a probe. Not a shovel. Most are pretty close to the surface, so in the summer the grass is just about dead over the septic tank. Look for the brown rectangle.
If you don't want to have to find it again in five years, call your local concrete products supplier. They probably make septic tanks right there. They will have access to a locking (screw down)replacement cover you can add to bring the access to the surface. For about $30.00 you can have a green plastic lid in the lawn instead of a hole every so often.
I agree, just use a probe. If you know where your line leaves the house you can tell how deep it is and you can follow the line with the probe if you don't know where the tank is. Or you can do it the hard way and dig down to the line and uncover it to the tank.