If your home is located outside the city or in a rural or remote area, where a municipal sewer system would be too expensive to build and maintain, you most likely will have a septic tank.
Another way to tell if you have a septic tank is to look for cylindrical lids or an unusual lump of earth on your property. These are typically indicators that your property has a septic system.
The simplest way to tell is to see if sewer fees are listed on your municipal tax account. If they are, that means you are connected to the municipal sewer network.