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In most localities , the Health Department will have a drawing or map of the location .

This map or drawing was typically required for the securing of the permit for a Septic Tank and accompaning Drain field

IF a map or location drawing of the septic tank is not available , we will typically look for the greenest spot of ground or grass in the lawn . This most green of spots is usually an indicator of the end of the drain field of the septic system and is usually within 10 or 15 feet of the actuall tank location sometimes a little less sometimes a little more , just depends on location and soil type . The septic tank is usually within 18-24 inches of the top of the ground and is sometimes located by pushing a steel rod into the soft soil above the tank if you have any idea as where it may be .DO NOT DRIVE THE STEEL ROD WITH A SLEDGE HAMMER because if you are over it , You can burst the tank very easily and this becomes an rather expensive repair scenario !

You'll know that you are on a tank as at repeated location at the approimate depth of 18-24 inches , by repeatedly striking the concrete top of the tank , hearing a THUD Sound and much resistance against the pushing of the rod .

Often ,actually locating it without any tale- tale signs requires much digging down to 24 inches in attempts to find it . We normally seek out the greenest softest spot of ground and work from that point.

If you are inexperienced in this , we highly suggest that you contact a Plumber or Septic Tank pump company . It is better to spend a couple of hundred dollars to locate the tank than it is to BUST a Tank , and have to spend several thousand to replace the tank and the drain field as well ! Either way you will still need to Pump the Tank out and that will generally cost between $ 200.00 & $ 400.00 depending on dumping fee's !

Best of Luck

Another couple of tips -

1) find interior sewer pipe direction to identify where it exits house walls - septic tank is usually straight on same line, even if leach field angles off. Many houses have a cleanout 2-6 feet outside the foundation - hopefully sticking above ground, usually 4" pipe of any color or material, usually with a black rubber cap or screw-in plug.

2) while grass is usually greener over the leach field, it is also commonly brownest over the septic tank if flat-topped concrete and quite shallow, rather than horizontally oriented cylindrical steel or plastic.

3) Turn off air conditioner if running and audible, and any other loud noises in the house. If you turn on a couple of indoor faucets, and take a standard stethoscope (about $10 at drugstore), push (don't drive to avoid damaging utilities or tank) a metal rod into the ground 6-12 inches and loosely hold your fingers in an "O" around it to stabilize but without grabbing onto it (which deadens the sound), hold the stethoscope on top of the rod and listen for water running. If you don't hear water running, someone can take a light wrench or screwdriver and rap the sewer pipe inside the house, or preferably at the cleanout outside - just don't whale on it so as to not crack the pipe. Then track to loudest sound point along the front of the house which will be over the pipe, then track out away from the house to the end of the sound where it is loudest, which will typically be where the water splashes into the tank, unless the sytem is backed up in which case it will be a matter of tracking to where the noise stops. That will be within a couple of feet of the closest end of the tank to the house, and near the inlet hatch. Typically about 4-7 feet further on in a straight line (usually) will be the outlet chamber hatch. Then probe around as per Ben's comment - tank may be plastic, fiberglass, steel, or concrete - hatches usually steel except on some plastic/fiberglass tanks, and stick up 6-12 inches above tank centerline and almost always centered on long axis, so outline the outer limits of the tank (or centerline (highest point) on cylindrical tanks), then check along centerline for 2 raised hatches and dig there. If you don't feel the hatches, dig at where sound stops to locate tank centerline, then along centerline away from that point to find the typically two hatches (occasionally one).

Sometimes a metal detector with a beeper or tone (stud finder type that finds nails and screws in walls) can find the hatches right off, if steel. Mine is a little handheld $10 one from Harbor Freight Tool designed to find only metal in walls, and can find metal pipes and manholes and survey corners at about 2 foot depth, so might be worth a try first if you have one - hold just high enough over mowed grass to not touch, as touching grass throws it off.

Usually about a 15 minute job with rod and stethoscope and shovel.

Note - if locating tank because of a backup situation in the house, use tapping instead of running water.

One other last minute thought - if you have the original survey plat for the property - maybe included in your purchase closing documents, commonly attached to the back of the Title Insurance, original as-built survey might show dimensions and location of tank and leach field. Sorry I did not think to say this before.

If you know when house was built, Building Department might have the original permit in the files with the planned location as well.