How do you know if you have a septic tank?

If you are in a city, it is unlikely you have a septic system. Country, you probably do. Most places have a charge for sewer on the water bill. If you are on a well, you have a septic. When it is dry in the summer, the grass over the lateral field will be green longer than the rest of the yard. Look to see where the main drain leaves the house. If it is towards the street you are on a sewer, if towards the side or back yard, it is most likely a septic.