Check your water bill. If there’s no sewer charge, you’re on septic. Or check the property: if you have a green/cap-style access lid in the yard, that’s a septic tank. Most rural Cincinnati and exurban properties are septic.
More detail
Three quick ways to confirm. (1) Water-and-sewer bill: municipal sewer customers pay a monthly sewer charge typically equal to or greater than the water charge; septic customers pay water only. The bill line item will say "Sewer Service" or similar. (2) Yard inspection: a green or black plastic riser lid (10-24 inch diameter) flush with grade indicates a septic tank below; a metal grate or concrete cleanout cap indicates municipal sewer. (3) County records: each county health department maintains records of permitted septic installations searchable by address. Roughly 60% of Greater Cincinnati housing on city water is also on city sewer; the remaining 40% (mostly older exurban single-family) is on septic. Most homes built after 1990 in unincorporated Hamilton County, rural Warren County, and Clermont County are on septic; municipal sewer expansion has been gradual but slow. Help with connection costs: the county health districts administer Ohio EPA Water Pollution Control Loan Fund money for failing systems, and in Hamilton County that money can cover an ordered sewer connection at 100, 85, or 50 percent depending on household income. Funding is limited and changes every year, so ask the health district and the sewer utility what is available before you budget.