Check with whoever issues permits for septic systems. There is often a plan on file, but they sometimes get changed in the field and not updated.

I'd start with checking your permitting office and see if they have the location with the permit for building the septic. If not check and see if it has who installed it and check with them. If not there check with someone that does septic installations and see if they can do it.

Because of sanitary regulations, there should be a permit and plan on file with whatever municipal agency monitors these things in your area. Septic systems are supposed to be inspected routinely....at least every three years but annually is best. Actual regulations governing septic systems will differ from location to location but there is some agency responsble for oversight and I'd start there.

We were given a copy of the as-built plans for our septic when we bought our house, they were on file with the town with all the other building permits. That would be my first stop.

We usually try to contact the person that installed the system

For a relatively newer home, the septic plans on file with the health department are pretty good. Go back to the 80s or beyond and it's a total crapshoot. We hit a line when digging for a pool because the drainfield was 500 ft from where the plans showed it (house built mid-80s).

I'd see what the health/building dept has first just as a starting point, but then call around to septic companies to find one who will do a locate. One here locally charges $250 to come out and mark the edges of the drainfield, the distribution box, and the tank.

"plans on file" doesn't always work with older systems.

Back to Mark's comments.

An excavator showed me how the lines went out from the tank, and ultimately we could follow each line due to slightly sunken soil from the original trench 60 years previous.

Do check with the permitting authority as the installation might be smaller than the space reserved, which is often 2x the sized needed, as they reserve replacement space that you cannot build on.

Look for a large rectangular area of green grass.

I don't know how installers locate them, but the folks who look for old outhouse pits to excavate for bottles use a length of 1/4" springy steel rod with a T handle at one end. Easy to push into the ground (may need to be wet depending on your soil) and you can easily feel different soils and of course pipes.

If you know of someone who wirches they should be able to show you. I have done it several times for others.

Even if you have a map on file, have a septic system specialist determine where it actually is. We were given a map of our septic system and drainfield when we bought our property. Since it was a few years before we built, we set up horse corrals according to the map. When we started the building process, our excavator used the "length of 1/4" springy steel rod with a T handle" and showed us that our map was entirely wrong. The tank and distribution box were where they were supposed to be (and rather obvious), but we had placed our horse corrals right over the drainfield. Fortunately, they are miniature horses, and the county's environmental systems expert determined that no harm was done. Also fortunately, this information allowed us to place our house in a much better location than we originally planned (and we had the acreage to change our plans).

Call a septic system installation contractor. Hire them to do a locate. They will come out with their probe and mark the field. It's not difficult. But, it's quicker and easier to hire someone who knows what they are looking for.

We have a Septic Pump Out company coming next week to tell us about our septic. They are doing an inspection which includes giving us all the information pertaining to the septic including the size, location assessment for leakage, Spaulding ,cracks, checks the filtering.
Our house is 60 years old and we did not know at the time to ask any questions or to have an inspection report included in the purchase of the house, there was a pump out included when we purchased the house.